7000 users/server is actually pretty high for an MMO, though I don't know how Zynga's MMOs compare to something like World of Warcraft in terms of server resources needed. The numbers being floated around for World of Warcraft are around 75,000 cores for 10m subscribers, or about 133 subscribers per core. Obviously those aren't single-core servers, but it's still under 1k per server. Of course this and the Zynga numbers are also both subscriber numbers, not peak player numbers, and I'm not sure how play frequency/duration differs between the two companies' userbases.
Some MMO designers actually really want to bring that number even lower, though lots of other people at game companies are looking at ways to pack more users on each core. But from a designer's perspective, if you typically get less than 1/50 of a CPU slice per user, maybe 1/100, and much of that is taken up by doing basic bookkeeping, it's hard to put in things that even 1990s RPGs took for granted, like NPCs with at least passably interesting AI.
Could you point out a WoW mechanic that is actually taxing? From what I understand, WoW AI is a height agnostic beeline and battles are heavily scripted at best. The tricky part seems to be the real-time player interaction a.k.a 3d chatroom, which has nothing to do with game mechanics (e.g., IMVU).
Many standard rpg mechanics in a large, seamless, 3d world full of both NPCs and players can tax resources.
Movement is one already mentioned, but in general, AI isn't only turned on in combat. NPCs move outside of combat - most creatures in the world wander about or follow a pre-defined path. I don't recall if they did on WoW, but I know in Everquest creatures used spells of their class both in and out of combat, such as buffs and heals - a creature in Everquest would even heal and buff up their allies. If monsters are buffing, you now are tracking those buffs & their durations - when we added monsters buffing to our multiplayer game, we made them infinite in duration to get acceptable performance.
Another operation that could be taxing is calculating line of sight (LOS) - this factors into any action an NPC or player would do. You can't have monsters attacking players, or buffing allies, through walls.
The problems movement incurs aren't "game mechanics" so much as "3d chatroom mechanics".
Buff tracking as a performance hit is surprising to me, though. I'd presume you were tracking monster lifespan - with this and an offset for 'AI interruptions' (like for chasing players as the buff wore off) you could recover the buff times and remaining durations without touching the monsters when they were out of player LOS. I can see LOS as a more taxing system, but monster LOS has no z-axis and the dungeons don't seem to require too many nodes for a pathing graph...
At any rate, most MMOs have rather... shallow... gameplay mechanics, and I don't think it's really computation that's holding them back.
The easiest mechanic to point out is movement, and the synchronization of movement. At every step in the game, I'm moving along what would traditionally be game tiles, in a given orientation at a given velocity. While the 'world' is rendered around me on my PC, the movement and orientation changes all have to be upstreamed to the server and calculated for normalcy (to detect speed hacks, cheats, mountain climbing, etc).
One key difference may be the sheer amount of metrics that Zynga tracks. I've heard they keep stats of every (however mundane) action each user performs. It racks up to a huge amount of data.
Some MMO designers actually really want to bring that number even lower, though lots of other people at game companies are looking at ways to pack more users on each core. But from a designer's perspective, if you typically get less than 1/50 of a CPU slice per user, maybe 1/100, and much of that is taken up by doing basic bookkeeping, it's hard to put in things that even 1990s RPGs took for granted, like NPCs with at least passably interesting AI.