This is a small guide to some of the more interesting changes made that should make you want to roll an alt rather than run another heroic!
Weapon Skills are removed
Yepp, you probably already know about this. But have you thought about what it really means? Really really? No more skilling weapons. No more having to go train to be able to use that axe. As of 4.0 all classes have max skill (for their level) on all the weapons their class can use. This is extremely welcome because weapon skill was one of those things that made leveling alts (especially melee alts) so tedious. Now whenever you find a new weapon you'll be able to use it at top efficiency at once! It is awesome.
Questing made even easier
Any mob that is part of a quest will now show their nameplates, making it extremely easy to easily identify what has to die without checking your quest log. No more killing the wrong mobs for 15 minutes before realizing your mistake. As soon as you've killed enough mobs for your quest, the nameplates go away.
Dual Spec costs
No I didn't miss to type a zero there. Blizzard have cut dual-spec cost into one
Cold Weather Flying is available from level 20
If this is a bug or actually works I don't know however. Does that mean we can fly around on slow mounts in Northrend from level 20? I don't know why anyone would want to do that, but it's damn cool if it's possible. You no longer need a tome to be able to buy it at level 70 anyway and it only costs 500g now!
Casters oom after every mob and melee can't handle more than one mob at a time due to lack of self-healing. That has long been the truth for most leveling classes. Blizzard have tweaked mana efficiency over the years, and damage output as well. They've now taken this a step further. Many mana users have become skills that are free of mana to use. Yes you read that right. Skills that cost no mana at all (but usually come with some prerequisite). I can't say if this is a fact for every mana user, but at least some of them. Alot of melee have become tools to allow them to heal themselves somewhat. Blood Craze for Fury warriors have become better and Recuperate for rogues allows for shorter downtimes between mobs. Nothing game-breaking of course, but it definitely makes alot of difference.
Now we get Crème de la Crème at level 10
This is my personal favorite. Every talent tree used to have that skill at the very end which was Blizzards way of rewarding you for going all the way down. Now that they are forcing us to finish one tree before we can go onto the next, they've decided to give us that talent with our very first talent. I'm not sure if you fully understand how awesome this is! It is like giving a shark-rocket with lasers to a 5 year old. Something will die! Here is a list on what you can expect to get on your level 10 alt - marvel at the possibilites;
Prot - Shield Slam: THE best tanking skill for warriors. Makes threat way less of a problem. Combine this with thunder clap and it's an aggro win.
Fury - Blood Thirst: The core ability of Fury (aside from Whirlwind). The healing part of this skill also makes leveling a little easier.
Arms - Mortal Strike: Imagine level 10 arms warriors running around in BGs with this baby. Quite annoying they would be.
Affliction - Unstable Affliction: All warlocks do good damage in lowbie instances now (this was definitely not the case pre-4.0). I am guessing this is one of the reasons.
Demonology - Felguard: I did Stockades with a warlock and her Felguard had done 42% of her damage. The only one doing more damage than her was the arms warrior. Getting your own über-pet at level 10 is too cool to be true. No more having to run around with that poor voidwalked for questing.
Destruction - Conflagrate: You'll probably two-shot most things at low levels with this thing.
Elemental - Thunderstorm: Thunderstorm aka thunderfuck is called so for a reason. It is great for questing, mana regeneration and pvping and getting it at level 10 is crazy.
Enhancement - Lava Lash: Pre-patch this would have been an insult to enhancement shamans. I haven't tried it but I've heard Lava Lash has been considerably buffed and now deals some 15-20% of total damage instead of the 3-4% it used to. It would've been even cooler to get Spirit Wolves at level 10, but I suppose that would be a little OP ;) (Worth noting also is that enhancement shamans get dual-wield at level 10)
Restoration - Earth Shield: Enter instance, put earth shield on tank, profit. Not to mention how great it is for questing since it reduces pushback by 30%.
Assassination - Mutilate: I often see mobs go from 80% to zero in instances. It is nearly always an assassination rogue mutilating them. Crazy.
Combat - Blade Flurry: The only real aoe-tool a rogue can have before FoK at level 80.
Subtlety - Shadowstep: The best tool for a leveling rogue imo.
Discipline - Penance: Penance at level 10! I don't even know what to say. Great for both questing and healing of course.
Holy - Holy Word: Chastise - One of the few "new" skill among these. It would've been really cool to get Guardian spirit at level 10, but maybe they thought a leveling priest would have more benefit from this skill. I agree. You won't get the most from this until you've specced Chakra and Revelations several levels later though.
Shadow - Mind Flay: This isn't as cool as the others since shadow priests already got mind flay fairly early before the patch. Shadow Form would've been neat! On the other hand, Mind Flay is the most important skill for a shadow priest (afaik), so getting it right away is nice.
Holy - Holy Shock: Holy Shock is extremely powerful at low levels. I don't know if they lowered mana cost on it or something but healing with this is extremely easy. It's also a great tool for questing and your most powerful damaging skill at low levels.
Protection - Avenger's Shield: I've always thought Avenger's Shield was among the best tanking skills in the game. It is ranged, it silences and it hits three targets. The awesomeness never seems to end. Now put this in the hands of a level 10 prot paladin and you do the maths.
Retribution - Templar's Verdict: Yet another new skill. It is like a Crusader Strike only it hits much harder.
Arcane - Arcane Barrage: I'm not sure if they've changed how this spell works in 4.0, but at least this makes leveling as arcane way less useless than it used to be.
Fire - Pyroblast: Pyroblast is crazy. In many ways. In fact I'm not so sure how useful this could be since it still has an insanely long cast time (3,5 sec) and costs alot of mana. This won't be really useful until you get the Hot Streak talent.
Frost - Water Elemental: Giving Water Elemental at level 10 to frost mages will only ensure the legacy of frost being the best leveling spec for mages. And also make frost mages awesome.
Beast Mastery - Intimidation: Since hunters are already OP, they're level 10 skills are a little less so than for the other classes. Intimidation is great for helping your pet hold aggro and interrupt caster, for instance. But not game-breaking like some of the other skills people get at level 10. Not that you'll notice any difference, you'll kick ass anyway.
Marksman - Aimed Shot: Doesn't lower healing taken anymore, so this is "just" a really powerful shot. And who doesn't want that?
Survival - Explosive Shot: Pre-4.0 this was the best damaging ability of survival hunters and the entire rotation revolved around using this as often as possible. So you can imagine how much fun you'll have with it at level 10.
Moonkin - Starsurge: Yet another new skill. It is like an extremely powerful Starfire. This hopefully makes lowbie moonkins a little less wrath-spammy.
Feral - Mangle: No more having to use useless skills as substitute while waiting for the real thing to come. Feral finally get the fun stuff at once (also worth noting is that feral cat shapeshift is available from level 10 now).
Restoration - Swiftmend: One of the drawbacks of lowbie restoration druids where their lack of a moderately sized, not extremely slow cast, healing spell. Swiftmend does have a cooldown but will in most cases probably be infinitely more effective than Healing Touch.
Blood - Heart Strike: Death Knights are a little special of course since they never actually are at level 10. If they haven't changed anything you still get talent points while questing as a death knight, which means you should get your first talent points pretty soon after creating your death knight. I can't say how important Heart Strike is for Blood Dk's now after the changes, I'm just assuming Blizzard have attempted to give dk's about similarly powerful starting tools as everyone else.
Frost - Frost Strike: See above.
Unholy - Scourge Strike: See above
So now is definitely the time to roll that alt you always wanted!