I like the fact that it is a good stress outlet. Also because it is a differant world than ours. Waterworld vs. air world. Also reminds me of having a tank when I was a kid a simpliar time.
Pretty much everything but color. I cannot stand bright fish with only a single (Yellow Lab Cichlids). If the fish has color patterns it is a bonus, but really not much of a deciding factor.
I think their personality gets my attention. My midas was interactive in the shop at about 2", while the rest of her siblings scattered when I went to the glass she stood her ground, almost like she was studying me like I was her. Really, I think she picked me.