Friendship instantly becomes a bigger challenge when life gets complicated by health issues. Only the best people will stick by you when these things happen.
Qualities I look for now are so different to what I used to look for in people (Less surface; more substance, these days!). But I have always thought that starting with shared values is a big place to begin.
A great big wide sense of humour helps an awful lot, too. Humility and empathy and a normal, healthy degree of selflessness helps.
After that, anything else that you can share as a pastime with people is always a plus!
I can only be an 'ok' friend, these days, I'm afraid. I have trust issues that stack up pretty high. And it's hard just to physically be available for people, as well. But I do what I can.