I like Robet Scoble's 11-point list about what makes someone a "follow-able" person online. Though his post was referring to his Twitter contacts, I thought that his points would apply to any of the other social networks I subscribe to, just as well .
You can see Scoble's original points about "followable" people here. Meanwhile, I paraphrased some of my own below to sync up with my sentiments about social network contacts in general. (You may notice my list is shorter than Scoble's by one line item. Mainly because I wasn't as stringent on criteria for people who've made something of themselves -- his item #3. Nothing wrong with it, it's just not as important to me right now.)
My Follow/No-follow Criteria
1. Vague bio, less inclined to follow you. I took time to post a descriptive bio for your convenience; my friends did, too. I would hope you'd see the value in extending us the same courtesy.
2. No picture, less inclined to follow you. This doesn't mean I won't follow you. It just means that the rest of you're online personality has to somehow explain why there isn't a pic of you today. Maybe you're new to social media and are still figuring it out. But, if the rest of you shows an inquisitive, eager to learn personality, I'm with ya. (But, you should quickly tune-in to the fact that, until you get a pic online, you bear a remarkable resemblance to many spammers.)
3. Sleazy or inappropriate. Oh, on a related point, if you have a sleazy picture (i.e., naked or nearly so), don't expect me to follow. Regardless of what the rest of you says.
4. If people I trust or respect swap updates with you, I'm likely to tune in, as well. It's a variation of the transitive property from basic Algebra. A likes B; B likes C; therefore A will likely play well with C.
5. Who do you follow? I may take a look at peeps you're connected to. If they seem like folks who are predisposed to engaging others--both online or offline--I'm with ya. After all, if they don't like to engage others, and you somehow share that sentiment, then what's the point?
6. If someone I respect tends to quote you or your posts, I'll tune in. It kinda plays to point #4 above. But, it also puts some of the onus on you. You have to make an effort to be engaging, post quality content, give valuable information away for free and not be a sales-pitcher. In other words, be a giver, not a taker.
7. If you seem to make things happen, I'll tune in. I personally suck at this. But, others are great at starting meetups, leading conferences, creating events. Though I suck at these things, I do well when I'm at the event. I like participating, meeting folks and fitting bodies to faces. So, if you're inclined to initiating meetups in my local area, I'll help you by helping you to fill your room and engage you and others who come to your event.
8. Are you associated with an interesting brand? Even if (perhaps especially if) you're a competitor, I'll follow you. That fact alone makes your brand interesting to me.
9. What do you talk about? If I look at your tweet stream, or you historical status updates and it's populated with sales pitches, you're probably a taker, not a giver. Sorry.
10. Have we met face-to-face at a meetup? This, to me, is the ideal next step in online social networking and constitutes an automatic follow from me. Live meetups to me is just as much about social networking as, well, what we'd otherwise think social networking is about (i.e., online). I don't think social networking is just about establishing a presence on Facebook, Twitter, LinkedIn or YouTube. It's also about brick-and-mortar meetups that, perhaps, got their start in one of the online networks. If we meet up at a meetup, let's swap cards. I'm likely to be one of your new followers the next day.
Those are the biggies for me. Did I miss anything? What would you add to the list?

