I'm a born in, so I can only go off of my observations. As far as those that I've seen convert in my experiences, it was mostly those ones who were a "little off" so to speak. When I say that, I don't necessarily mean crazy, but rather they often were a little left or right of society's norms. I've seen some educated individuals convert, a couple of whom are quite respected in their fields of occupation. With all of them I've noticed however, regardless of the educational background or current economic status, they all carried some moderate to heavy emotional baggage. In times past and even nowadays, when people were having problems or "going through it", they often turned to their churches which they saw as turning to God. With JWs going door to door, and as often as they do, they're bound to show up on the porch of someone who's emotionally vulnerable.
My own father, was a convert. Let him tell it, he converted just to marry my mother, but that's not entirely the truth. His upbringing was rough, and his siblings, my uncles and aunts, are all pretty jacked up mentally. He's the only one stable, and even thats a matter of opinion. The era he grew up in was hard on black males, not saying its still not effed up, but not as bad as it was during his teenager and young man years. He explored various religions, and from what he's told me, he experienced racism and segregation with various churches back then. Eventually he ended up studying with the Nation of Islam, but he had enough sense to realize that they're crazy, and really nothing more than a "social" religion for those that catch my drift. Besides he didn't need to hang out with other blacks as an organization to hate white people. Another thing was, he said with the JWs, it was the first time that anyone actually made an effort to explain to him whats in the Bible. Also, the racial tensions and segregation that this country was dealing with at the time, wasn't as overt or even non-existant to some degree amongst the Kingdom Halls he'd been to. That's not to say there wasn't some racial problems in congregations, but it was more subtle. It wasn't as overt as what he experienced when white churches would turn him away at the front door as if he as Judas Iscariot reincarnated in the flesh. Last but not least, he met my mother, and from there it's been a wrap. They've been together for 5 decades, and still act like they're dating.
Speaking on my dad converting partially over my mother, I think quite a few JWs have converted for that reason too. Whether it be the wife, or husband who initially studied and got baptized, somewhere down the road enough "unbelieving mates" have converted, or have shown potential that the WT has printed numerous articles about how the believing mates conduct won over their spouse.
One last thing, I think it goes without saying that immigrants and foriegn populations are ripe for JWs, much in the same way the Catholic and Protestant churches converted millions through missionary work, when they weren't converting them via violence.