I'm 5'5 and a half and I haven't had terrible problems seeing when I have been a few rows from the front--really just depends on who is in front of you. Regarding the knee issue: you should be prepared, at a stadium show, to stand for at least four hours. You can sit between the opener and U2, and sit right up until the opener if people around you will tolerate it, but really, there's just lots of standing. Also, you'll be standing for at least an hour when the GA line gets ready to go into the venue. If your knee causes you pain when you stand for long periods, then obviously you should consider that.
People who don't do well in the GA: those who can't stand crowds (duh, I know that is rather obvious), people who have a hard time tolerating heat, and people who are used to a pretty sedentary lifestyle. I've never been shoved or pushed much (except at the opener in San Diego) so that has not been a big issue. You should be prepared to get kinda dirty at an outdoor show...lots of stepping through spilled beer and god-knows-what on the way out, not very clean places to sit (bring a trash bag wadded up in your pocket--good for rain or sitting). All in all, I love GA shows. Can't imagine not doing it, even though it's exhausting!