First of all it is good to know that servers are like persons. No one server is the other one alike. One of the most interesting things about occasionally playing on another server than your main is to meet new people and especially how some servers use completely different systems for doing things than others. If you're not ready for this you can get yourself into some real interesting situations. For example; Most servers use some sort of score system to rate players gear. My server uses the good old Gearscore addon, which means a regular advertisement for people to weekly would look something like "LF3M to weekly, whisper achis and at least 5k gs". Ok so most weeklys don't require good gear, but merely that you know the tactics, but that's beside the point. To me this is the "normal" way to ask people to come well equipped. I won't debate whether gearscore is a good or bad addon, I've done that already. People will want you to be prepared for the raid, and will find out about it one way or another. Gearscore is one way.
But then I rolled an alt on a completely different server and saw a message in trade like so; "Tank LF raid to ICC, got 3k gs and know tacs". I immediately reacted, because 3k gs is really, really low. It is less than you'd expect from a freshly dinged, and badly geared, level 80. I found it completely hilarious that someone would advertise for a raid, any raid actually, with such a bad gear. At first I thought this was a mistake or a joke. But people kept on advertising their low scores when asking for raids. Finally I told them; "umm... 3k gs is extremely low, why would anyone let you raid with such a bad gear?". The answers I got were interesting but mostly consisted of "3k gs is pretty good actually.". At first I didn't understand anything. I had clearly misunderstood something. Turns out this server wasn't actually using Gearscore, although they called it that. They were using WoW-Heroes Score. And since WoW-Heroes are using a completely different scoring system, 3k actually is pretty good for most raids.
Me coming from a completely different server, using another system, didn't get this at first. And because of this I made a fool of myself in trade (although I don't understand why they'd call it GS when it clearly should be something like WHS). Actually WHS (WoW-Heroes Score) is a better scoring system than GS since it takes into account gemming and enchanting, which GS doesn't. But the interesting point here is that coming as an outsider there were new rules for me to learn, new ways to do things. And I had to learn them to get around properly on this server. If I had advertised my gear as Gearscore, people would've cared less as Gearscore doesn't say enough about your gear. If someone had used WHS on my server (calling it gs) they would've been the laughing stock of the day (or maybe week).
And this is far from the only rule that differs from server to server. I remember when I did my first server-jump together with some friends. This was some 5 years ago. We thought we had ended up in twilight zone since our new server had the weirdest of rules. I don't remember them exactly but I recall it being rules about what was ok to need on (which differed completely from my old server), how to complete instances and things like that. And still today I sometimes join a group where someone questions the way I handle something in an instance. "We always go this way" they say. "Well I've always gone the other way" I say. Now that pugging means being thrown in with strangers from other servers no matter which server you're on, your pugging experience won't differ from being on an off-server. But your questing or raiding experience might and knowing about these differences might be useful. They're definitely very interesting to find out about anyway. "Hmm, so you do this that way? Never thought of that...".
Some others things that affect your gameplay when playing on a server other than your main are a little more obvious, but maybe not until you've started playing and realize - you're really all alone on this server.
You don't have your friends and you don't have your own resources. If this is your very first char on this server you won't have any money to buy fancy gear or maybe even skills the first levels. When I reroll a char on a new server I actually usually have to vendor my starting gear to afford the first set of skills. Getting money as a lowbie is actually fairly easy and you'll quickly afford not only your skills but also alot of nice gear if you like, especially if you follow the tips in the post I wrote about it. But unless you start AH-camping you won't quickly be so rich so that you can afford anything. I'm not rich on my main, but if I see some epic I want I've generally got enough money to buy it. This is definitely not the case when being off-server, and especially not once you've finally dinged your alt to endgame and want to buy crafted gear for thousands of gold. Just buying Epic Flying might be the investment for several weeks to come.
Not having your own resources usually also means you won't have access to any BoA gear. And let me say this at once, anyone who claims BoA gear doesn't make much of a difference is a big fat liar. The 20% extra experience (In Cataclysm you might even get up to 50% extra experience through BoA gear) is the obvious part. But the BoA gear usually also has a really good itemization for your class (given you've chosen the right pieces and aren't playing a tank) and often convert to what is the best gear for those levels in terms of amount of stats. Sure this differs between classes and actually nowadays few classes have big problems with leveling anylonger. But I've compared a BoA geared and non-BoA geared char of equal level and class and the difference is huge. Measured in killing effectiviness I'd say a BoA geared char easily kills twice as fast as a non-BoA geared, counting more survivability, extra damage and less downtimes. Like I said, leveling without BoA-gear is still easy. But BoA gear sure gives an edge. Not having it makes a difference, no doubt about that.
Not having high leveled friends around means you won't get any boosts. This has several implications. If you really want a certain gear piece from a certain instance there are only two ways to get it, and they're both way more tedious than having someone boost you for it. You can either a) go get it yourself, which isn't even a possibility for many classes, and/or you might have to wait till you're high level enough to pull it off to make the item you want less interesting. Or b) random pug for that specific instance, potentially loosing it to other needers (which might also be ninjas, which just makes me go "yay" inside). There is an option c) pay someone to boost you. But because of the abovementioned issue with getting huge amounts of money fast, and boosters generally asking for alot of money for their time and effort (easily 10g for a run somewhere, unless you can find a bored kid who just rolled a dk and wants to pwn lowbies in an instance), few people actually think a certain item is worth it.
Not being boosted does create problems all the way up to endgame. In fact I think that in endgame not having any contacts really begin to become an issue. When you know people on a server they know what good you are and might bring you for raids eventhough you're undergeared. Love got to join for an ICC25man raid with his newly dinged warrior. Sure, it was a gdkp run and people were hoping to make some nice money of him, but him knowing the raid leader surely played its part. And I know from first hand experience how extremely difficult it is to get into raids when you're all alone and have no one to back your credentials. People want achievements. You need to raid to get those achievements. The tougher achievements will be only be gained either by being lucky enough to get into a pug (which doesn't happen often), or by having friends take you. On most of my off-servers characters I've really have to bust my ass off to get into simple raids like VoA and some tougher weeklys, even with 5k gs. Admittedly it was as a tank, and as such it is more important than for any other class to have a nice gear, but still. People don't want to bring my 5k gs geared tank for weeklies like Jaraxxus. People weren't anywhere near 5k gs when that instance was first launched! But people want to take the smooth ride, and I don't blame them at all. But it means that you'll have to work so much harder to prove your worth to get to join a raid, and often not even being given the chance at all.
You Vs The Pug
No friends also means you have to either quest all the time or accept the possibly infuriating random-pugs. To me, random pugging has turned into something of a lottery, but one with the chance of loosing money as well as winning some. Because pugging rarely leaves you in a neutral emotional position. Either you find a great group, and get so happily suprised that you love the world. Or you end up with 1-4 complete asshats, which get you so annoyed you want to punch them through the screen. Interestingly enough I often end up with groups that consist a little bit of both. And with a lot of meditation, finding my inner peace and following my pointers on how to survive the random pug I nowadays rarely get out of a pug angry, as I often did before.
One possibly good thing about being all alone on a server is that no one will bother you. If you're one who loves to chat and comment everything you do while playing this could be a problem, but you could always join some guild on your off-server to solve this of course. I rarely do since I can vent all my joys, frustrations and thoughts on Love and so I don't feel left out even when I am playing alone. Also now that the new friend system lets you chat cross-server this has definitely become less of a problem. But if you're an officer or a guild leader or otherwise often have a lot of people who ask for your attention when you're playing on your main or on your main server, playing on an off-server can really come as a peaceful vacation (funny since all of WoW should be relaxing really).
Not having anyone but yourself to rely on can really come as a fun challenge as well. Having to start everything from scratch could sound scary or tedious, but if you succeed it's often more rewarding. Not having BoA means you'll be all the more happy with some crappy blue. Not having friends to boost you will make you the more pleased when you kill that nasty elite all by yourself. One could argue if it's really happyness when you get happy about something crappy just because you can't reach what is really good. But I think as long as you're having fun that philosophical blahblah really doesn't matter.
So to conclude, playing on an off-server means;
- New "social" rules to learn
- No boosts (unless you pay a hefty price)
- Not being able to buy anything you want
- Accepting defeat and your limitations
- Being a social outcast (at least at a beginning)