Complete lack of any AI for the monsters

[fractaled]

(Up front I'd like to say I'm not disagreeing with your basic premise that this would require a total rebalancing, not to mention other things)

1) Colission detection would be required to keep mobs off healers. In that case tanking skills are not required at all and it is about finding some choke points, so the mobs can't run around the tank

I think collision detection would add a great deal of tactics to a game. The choke points, while lending an advantage to the holder, probably wouldn't be ubiquitous, and probably wouldn't be necessary in most cases. In skirmishes between 5 players and 5 monsters, 4 non squishies could effect a wall around the healer. Also, if non-frontal or non-engaged attacks did significantly greater damage, it would not be feasible to run around 4 other guys to get at the 5th.

2) After entering any instance with somewhat organized beings, like Deadmines, SFK, SM, Strat/Scholo/BRS/DM one or two mobs will run away and call back everyone in the instance to kill you. It should actually work for any place in the game - such as Scarlet Crusade fields in Tirisfal Glades when leveling from lvl 1 to 10 and many others.

This would be quite the thorn for level designers. Snaring/silencing/taunting would be very important. Ambushing/trapping would also be more important.

3) All bosses with AoE would just run into thickest pack of players, preferably with healers and spam AoE until they are dead

I think there is something wrong with "running into the thickest pack of players". This doesn't happen in real battles (unless you're a suicide bomber) because it exposes you to attack from all sides, some of which are typically vulnerable.

4) Drak would Conflag and attack healers right first and his guards would focus fire on them. He would definitely not allow to be hunter-kited to the Beast room

5) Intelligent Onyxia fight would look like this: She goes airborne right in the begining and does Deep Breath (which usually wipes about 20 people) 2-3x times in a row and then deal quickly with the rest

6) Intelligent Ragnaros would first kill all ranged healing and DPS with his 6K damage ranged attack (atm he does that only when not occupied in melee) and then deal with melee buggers, preferably by spamming his AoE
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These last 3 would require the toning down of boss's abilities. Insane abilities like that are to cover up for weak AI. (Not that 40 people should necessarily be able to take down a dragon or a god anyway).