You must take into account that collision MUST be done server side or expect hack city. If it's done client side, anyone could hack it so you always get "head shots" or w/e. So, imagine a 50 vs 50, 100 vs 100 battle with the server trying to calculate all those hit boxes...
Could you go into detail addressing contemporary processors, collision detection calculation and implementation strategies, and network strategy, about why this would be difficult to do? It's not obvious to me.
edit:
And to answer the OP question, my regular spiel:
You need the ability to enact consequences on people on a personal (open PVP with looting) and guild (sieging) level. Everything else is essentially fluff and follows from that.
On a lower level, I'd like all the usual perks -- collision detection, aimed attacks, ballistics with bullets/arrows/spells where appropriate, skill-based system, considerable player skill importance, etc.
As far as minute things, I want fun stuff like
Creative crowd control!
- Summoning a tree wall to block your caster from getting raped by melee, instead of mashing your stun hotkey like a fucking monkey
- Specific counterspells and spell interrupts to stop your team from getting raped by an AE spell, instead of mashing your stun hotkey like a fucking monkey
- Freezing the ground to make people slip if they try to turn while running at you, and then running perpendicular to them to avoid them, to stop their zerg from surrounding you and raping you, instead of mashing your stun hotkey like a fucking monkey
If there are "epic" PvE encounters, let them be legitimately challenging. This means:
- have them use tactics instead of giving them 10x hp and 10x damage and calling it a day
- have them use their powers believably, instead of having them only decide to use their super power to kill everyone in your raid party if you happen to be clustered together (and then balance the power accordingly) -- a four-year-old controlling any raid boss in WoW could probably take out every WoW character ever created on all servers simultaneously
A fleshed out counterspell system!
- various counterspell stances that let you either interrupt the enemy's spell with your counterspell, or protect yourself from a specific spell with a "defensive" counterspell
- specific counterspells for each spell, based on other spells. "Counterspell mode fireball" counters "Ice bolt"; "Counterspell mode Haste" counters "Slow" and so on
- a way to identify what spells are being cast by physical casting gestures and power words (a la Asheron's Call)
- an extremely robust tree-based hotkey system (like the voice macro system in Tribes) to choose a spell to cast so you can react almost instantly if you realize what spell is being cast
You should never have to fight the interface to get shit done
- spellcasting etc. should be made as quick and easy as possible (see above)
- being able to click on dudes or hotkeys or whatever with great speed should not be a factor in PvP (Blizzard pulls this bullshit with Starcraft, where "clicks per minute" is a very accurate measure of "skill", and this is artificially imposed by stupidity like only being able to select 12 guys at once)
- I shouldn't get stuck on tree branches and fences and chairs that my dude will obviously walk through/over/around trivially without thinking
