Extremely slow death-gryphons
Problem: Whenever you die and have to fly somewhere, which most often is when raiding, you either start out alive with 1% hp (which is lucky unless someone happens to be ganking people at the graveyard) or on the regular death-gryphon. But wait, is there something wrong with it? It flies extremely slow for some reason, slower than run speed in fact (it definitely feels like it anyway). This is not what you want to be stuck doing just after the 10th wipe on some boss you're already bored to tears doing in ICC. I can imagine many guilds post-poning any raiding until Cataclysm just because of this neat little bug.
Solution: As far as I know there is really nothing you can do about it. My suggestion is you have some tv-series ready to watch, or scarf to knit, or book to read etc so that everytime you die and have to do that slow-ass flying again you can watch a bit on your series/do something else instead of nerdrage. That way dying won't be as horribly infuriating. Oh, and don't forget to keep soulstones up.
Crashing in instances and raids (and bgs) FIXED
Problem: Randomly (?) crashing upon entering an instance/raid/bg. It can happen after various amounts of time and doesn't seem to have any logic to it. Once crashed it usually doesn't work to log into that instance at all again. Until a couple of hours later.
Solution: An old guildie of mine noted that there is word going around that the random crashes are due to the CWOD - Cog-Wheel of Death. Whenever you mouseover something that is interactable through a cog-wheel, like the pumpkin on Headless Horseman, a mage-table or the like, you have a chance of crashing. Something isn't right about those cog-wheels right now apparently so when they try to send their data to you something has a big chance of going horribly wrong. I tried this myself by mouse-overing the pumpkin on Headless Horseman. It gave me a short freeze-lag, but no crash fortunately. But probably could've just as well. Thinking back on everytime I've crashed so far it was quite likely that I was mouse-overing some cog-wheel. So don't mouse-over the cog-wheels! Let someone else do it for you. If you want mage-food, let someone else trade you and etc. In the above link you can read more about the issue and find macros that will remove tooltips from appearing, which apparently will remove the crashing. I'll repost it here as well, so you don't have to dig it out, but all thanks goes to OP Deyneth. This macro will remove the tooltip when clicked by left mouse button and make it come back when clicked by right mouse button. I haven't tried it myself though.
/script GameTooltip.Temphide = function() GameTooltip:Hide() end; GameTooltip:SetScript("OnShow", GameTooltip.Temphide)
/script GameTooltip:SetScript("OnShow", GameTooltip.Show);
Nameplates are acting odd - PARTIALLY FIXED
Problem: There are really two problems here - 1. Nameplates don't show and 2. Nameplates show randomly. Although I think they have the same cause, they're not the same problem. Also they affect two different forms of nameplates. The first is the regular nameplates you can see over everyones heads like name, title and guild. The other is the V-key nameplates that show hp and threat for example. The reason titles, names and guilds have stopped showing might be because Blizzard have implemented a new and easier system for detecting npcs that are worth interacting with. Class Trainers, Quest Mobs, Quest Givers and the like are all npc's that will show their nameplates so that you can easily find them among others. Apparently they've made names/titles/guilds not show as a default. This works really well actually. But if you never quest this might be less valuable. The second problem might be due to the above changes Blizzard made to nameplates. It makes V-key nameplates randomly appearing and disappearing, which is extremely annoying if you're a tank and melee.
Solution: For the first problem all you need to do is go into Interface - Names and check the boxes that correspond with the information you'd like to see.
The second problem about V-key nameplates acting like jojos has another solution however. Type /console bloattext 1 in your chat and it should make them act less funny.
I can't see ground textures!
Problem: Although you've got "Projected Textures" on high as always, it seems like you simply can't see ground effects anymore. All you can see is a hint that something good/bad is beneath your feet, at best. This can turn out to be extremely dangerous in some situations where separating friend from foe among ground effects is crucial for your survival. Or where a ground effect can kill you in one shot, like the frost bomb on Mimiron (as I noticed).
Solution: Since there are so many ground effects going around nowadays, Blizzard thought they'd give us the chance to see our own effects better than everyone elses. So they implemented something called "Emphasize My Spell Effects" which will highlight your effects over others. But it seems like this doesn't highlight your effects, it just simply removes the ground effects of everyone else (and your own skills) right now, making it impossible to see what's on the ground. Just go into Interface - Display and un-check "Emphasize My Spell Effects" and the problem should be solved, it was for me anyway. Thanks to my guildie Patrock for this tip!
More might be added (although I hope not)!