Since 4.0, there have been a ton of changes to the interface and the way things operate in-game. The main complaint/issue I'm seeing is with threat generation and new difficulties for tanks to overcome.
First, let me state that I do not use add-ons. Not any. I did use Auctioneer and Altoholic, but since my last install of Altoholic (I have 5 computers that I may play Warcraft on) shortly after is when my account was hacked, so I will not be using any add-ons ever again. Which also means i don't have anything that tells me exactly my DPS or my threat generation. However, I do have a Warrior tank at level 56, and I've mostly leveled him by insta-queing instances. Currently he is operating in Dire Maul, which is a pretty good dungeon to be in. (Libram of Rapidity Ah's for 200-300g! I NEED on it, cuz i'm a jerk like that!) Also a great dungeon to run if you are a Warlock because there are very strong demons you can command for a change of pace from the imp or Fel Stalker, there are a few Warlock only quests, and Pimgib the Imp has a unique drop.
First thing I did when dropped into Dire Maul East is state to everyone that this was my first try tanking after the patch. People said that's fine (or nothing at all... damned anti-socials) and we went to town. First thing i noticed is rage was just flooding into me... NO trouble at all having enough rage to do anything, even just spamming Cleave/HS, I just could not get below 50%! Also, Victory rush. Warrior healing? On top of amazing armor and stamina? Holy Crap! Without even being healed, I barely took any damage at all!
The problem I noticed was that I was struggling my ass off to keep the enemies focused on me. Bloodrage is gone, so now my rotation starts with Charge --->Thunderclap--->Demo Shout--->Cleave to get as much AOE as possible, then tab-attack everyone in the group with Shield Slam and/or Devastate. Always worked just fine in the past, rarely lost any, and if i did i just rotated and taunted, no problem. This time however, plant-monsters were just streaming right past me to the lesser ranks; DK, mage and Hunter, not so much the healer, but he took some too... I was running all over trying to pick up the adds so no one took too much damage, then another one would peel off and I'd have to zip over to him. It sucked, but we didn't wipe... well, I died at one point, but the DK stepped in and they finished the encounter.
At first I thought the issue was the DK, who couldn't keep his finger off of the Death Grip button, but after reading a post by Tobold, I think it was more an issue with the group mentality. People USED to wait 3 seconds for the tank to get aggro then charge in... well, we haven't really had to do that in awhile, with gear being so prolific now and threat being what it was. I noticed it some on my Shaman in SM Graveyard; I was able to pull pretty readily from the tank. Luckily I'm enhancement, I can take a punch as well as deal one out, but Shaman's used to have abilities that specifically reduced their threat level, and now i don't.
Tobold mention a change to a more difficult Warcraft, where we cannot just AOE things down and have to use more of our tools instead of Facerolling through dungeons just to level. I, for one, would LOVE that, as I'm a career leveller and always design my toons with multiple functions in mind. Nothing satisfies me more than when the healer goes down and my Shammy can back up from thumping bad guys to heal the tank for the last few seconds to save the day. I see these changes as a chance to relearn to play a game that I love, and make things new! I've run all parts of Dire Maul several times, but now that my mechanics are changed, that makes the bosses and battles different than before, and therefore more exciting!
One other thing that may come up as a result of this, however. I mentioned before that I run my tank when I have plenty of time available and I feel like working and paying attention. Well, now that its harder, i may have less time or inclination to bother with doing that, making tanks even more scarce than they are now. Tanking is work, especially in LFG, because you never know if you'll get a veteran group and there is a high likelihood of douchebaggery going on. One jerk out of 5 makes the game 20% less fun, and as a tank, your job is tough enough.
I'll post more of my findings here later on, probably won't be playing for a few days, got personal stuff going on... BOOOO. If you have similar stories, give me a holler.