I'm of the mindset that Alts are primarily un... categorizable?
Ava/Kyllsa are fun to interact with and I generally have good feelings come from that one. Another is someone who has a male/female version of the same character; a nearly identical genetic structure, but two different people. Arista's alt (the only one I'm aware of, at least) varies so differently from Arista that interacting with him feels as if there's someone else behind the seat, making it an enjoyable experience.
I don't like to name names, for the most part. However, there's one specific person who has a horde of RP alts who will bombard the IGS with this same-family mob and clog traffic RPing with themselves. That's the kind of stuff that really bugs me. That, and the alts who are insisted on being taken seriously but their only purpose is for repping the Main's reputation or for something dinky and silly. You don't see an EOH character trying to be relevant in the Summit often, really.
Also, I second the notion of having an alt nobody knows about. Who knows: maybe Charles is the alt.
[spoiler]Spooky.[/spoiler]