The performance of a game does not usually get bottlenecked by poly count first. This is one of the reasons that pixel shaders have been gaining speed faster than vertex shaders over the past several years.
To put it somewhat bluntly: If you are asking about how many polygons your models should have, then you should not be working on an MMO.
MMO production has bankrupted large corporations, yet in every game development community (XNA forums, gamedev.net, gamedev.SE,...) there seems to be a weekly newcomer asking about how to fix some widget in the MMO he's making as a one man show.
There actually have been a few successful MMOs that started as one man projects, however they were all created by game developers with a lot of experience.
Studies have shown that players notice and care more about shadows, lighting, and aliasing before they care about poly count. Look at minecraft.
You can get away with very low polygon counts if your lighting effects are excellent. Global illumination approximations, shadow quality, antialiasing post processes, deferred lighting pipelines, should all be a much greater concern than polygon count.
Since your question is about polycount, I'll point out that (new) critters in wow are typically in the range of 50-100 polygons.
Mobs are in the 300-500 polygon range.
CPU speed is typically a bottleneck for most wow players in my experience though, especially in cities where the number individual objects (and therefore the number of draw calls gets large).