10) Often isolated or less social, particularly in crowds or large events. Face it, he's probably not going to be the life of the party. Unless it's a cocktail party for physicists, in which case I hope you like physics puns...
9) Can, at times, be an inflexible, insufferable know-it-all. You're just going to have to get used to it. Just remember, even when he thinks he knows, doesn't mean he's right. You can always remind him that science changes its mind about every 50 years about what's right, but that may just spark more debate.
8) May not always have great social skills or tact. But don't worry, he can learn. It just requires some patience and you have to explain it to him rationally. If there's logic involved, it can be learned and absorbed.
7) Sometimes, they can be difficult to understand. If you know half the words he's using, it's hard to follow. If there are offhand references to theories, hobbies or knowledge that you don't have, it's just awkward and slightly embarrassing. There's only so many times you can stop someone mid-point to clarify what they mean and have them explain something to you.
6) Less likely to ask you out. Afraid of rejection, as we all are. But, perhaps a tad more sensitive, owing to repeated previous social/romantic rejections. With a bit of extra/special attention, smiling and perhaps being a bit more obvious about how you feel about him (if you feel that way), he'll get there. Patience is a virtue...
5) Often present as unconfident. See above. He's probably plenty confident about some things, but unfortunately that's less likely to include being around beautiful women or striking up conversations out of thin air. There's confidence somewhere, even if you have to dig to find out he can build his own generator from a potato and some tin foil.
4) Less inclined to approach others, often at a loss for what to talk about. See above. Also nervousness, and anticipated/expected rejection. Sometimes it just takes a bit more sensitivity, acceptance and warmth. Other times, it will take a whole lot more time.
3) Won't always be assertive or straightforward. It didn't work in grade school, middle school or high school, so why would they expect it to work with women later on? You might have to let him know that you want to hear it, and being gentle/responsive when he is being assertive will go a long way.
2) Tend to be afraid of hurting feelings -afraid of making you angry or upset. These
guys have probably heard one too many times that they should never,
ever hurt a gal's feelings. And they took it seriously. Unfortunately, I think this is one of the larger issues, because it leads to...
1) Emotionally unexciting. Being afraid to slightly offend can cramp interactions, especially since being playful is about the little ups and downs, poking and jabbing and teasing mixed with joking and compliments. That kind of banter is often the essence of flirtatious attraction.
Perhaps that's the biggest point -not being afraid to be playful. Find a playful nerdy guy, and you're set. Or perhaps you can encourage it (eg. being playful yourself) and see whether or not it comes once he relaxes.