Here's an article on MSN careers on the difference between being "nice" and "too nice" (a pushover). I've never been accused of being a pushover. But early in my career (ironically, when I worked for the company that produced the article), I have to admit that the self-imposed need to prove myself to my manager and colleagues did cause some later-than-necessary evenings in the office. I did it to myself...<yawn>.
For me, it was less a function of my general disposition (the snark has always been there, folks) than a lack of confidence and maturity (and experience, obviously). Having said that, I don't think there's anything wrong with working hard and "paying your dues" early in your career. It's kind of like finding your sea legs. You have to get past it to survive.
I've found that I tend to thrive in work environments where spirited discussion is well regarded. Though it is great to have everyone agree with everything that comes out of your mouth (as if that ever happens), bouncing around ideas and opinions is fun. And it helps you make better decisions. It's outside some peoples' comfort zones and can take some getting used to. Acceptance of that kind of conversation has a lot to do with corporate culture (one of the reasons why Microsoft works for me).
And who isn't on the look-out for co-workers engaging a pushover? I know that I have experienced the realization that someone is taking advantage of me (in my case at the time, my relative lack of experience and need to prove myself). It's hard to balance collaboration and saying no (one of the most underrated skills in the workplace). Again, a matter of business maturity and confidence. Hey, the more successful you are, the more people will come out of the woodwork to ask you for stuff. It's good to have something to say no to.