(My project is at http://wowqu.com - it's quite a bit more domain-specific, but it's been a lot of fun anyhow. :)
The "Questions, People, Ask a Question" are kind of the same way. The white font looks a little off -- I might try a slightly softer off-white color, but it also makes them stand out. A more experienced UI guy could tell you more about it, certainly.
The login and signup aren't nearly prominent enough, I think. I had a hard time finding them. If I played with the app more, I might discover that you've solved the problem by prompting for login/registration on actions, which is where I think they ought to be, or something, but they're also a little blah.
Lastly (and this is actually parallel to the adwords, but I'm throwing it in because you asked) the double-turn-down effect on the search just bugs me. I don't know why. If it were rounded on both ends, I'd probably be fine. If they turned in opposite directions (ala Digg) I'd probably be fine. It also doesn't scream 'search' to me either.
These might be the amateurish ramblings of a novice UI guy, so take them all with a grain of salt, but I'd at least try to get some consultation with somebody with more experience.
I really do love the layout, so please don't take any offense. I only say because you asked, and I really think the overall design is nice, even if you hadn't said you weren't a designer. The colors work, the transition from blue to white in the content layer is solid. The wraparound labels on the informational boxes may as well be gold-plated. The 'answers' and 'votes' buttons are splendid too. Like I said, I like it.
The nav items up there are definitely not optimal. I've been feeling that, too, but I haven't found the right solution to them yet.
Login and signup could be a lot more prominent, definitely.
The search box style is a rendering bug, due to how different browsers handle border-radius; I have that set to 15px, which produces nice round ends in Firefox, but Webkit renders 15px of rounding on the top corners, then tries to do the bottom corners and doesn't have any room left. It's on the to-fix list.
I really appreciate your feedback. I'm as much a novice as anyone here, and I absolutely recognize that I'm probably breaking about sixty-six critical laws of design somewhere, so feedback is good. I very much appreciate constructive criticism - thank you, very much.