Like stated above every server has its ups and downs.

For iEra I hate how the guns cost an extreme amount of gralats while the jobs there are terribly balanced.
Favoritism would probably be another thing.

For iZone I'd say the way staff have handled things in the past, but I think they have gotten better. The tileset could use a huge revamp. I don't mean a change in colors either. I mean an entire new tileset. Cant think of anything else since I don't play it anymore.

Since you did mention playercount I'd say that'd go for Ol' West. Usually on weekends we hit around 500+ and weekdays 300-400, but lately it's been in the 300s due to school exams and whatnot. But I'd say the thing that bothers me right now is how players wont give it a shot again, especially since the last time most tried it out was probably on release and a ton of new things has been released since then. (Outside tiles are being revamped, inside is done.)

I can't think of much for iClassic other than I see a lot of threads about how they lack new content.

Delteria.. well, wheres the iOS at man.