As one of the aforementioned 100+ character altaholics I absolutely recommend CoH. It's a very good game with an excellent community. Soloing is viable for just about any character (some more than others of course, but there's only a handful of powerset combinations that really can't handle soloing) and if you're used to LFG in other MMOs you'll find CoH quite refreshing because most players are not enslaved to the 'holy trinity' of tank/healer/dps.
One thing to bear in mind - the hero content, or blueside, is generally quite old, and not really as polished as the newer stuff. Villains, aka redside, has shorter, tighter mission arcs and better writing. Praetoria, the newest expansion, has the best writing and really good mission design, but it can be a lot tougher than the other stuff.
Anyway, villains are best. Robbing banks and beating up cops and heroes never gets old.