> "You've got to spend a lot of time there to get the ability to answer a question or upgrade/downgrade answers."
You really don't. You need 15 rep to vote up an answer, which entails posting one good question or one good answer (you need more rep, 125, to vote down, but you also lose rep by voting down answers, so people worried about their rep don't do that very often anyway). Anyway, just downvoting isn't very useful - a much more useful contribution, if you see an answer that's incorrect, would be to post your own, more correct answer and/or post a comment on the incorrect answer indicating why it's incorrect. Of course, you do need 50 rep to comment, which is sort of annoying, as commenting is very useful (though not *essential*), and 50 rep isn't that much, but it's not nothing.
You don't need any rep to post answers, as that's sort of the whole point of the site, and the main way to *acquire* rep in the first place.
Yes, I absolutely agree that as SO got increasingly popular, it also got increasingly deluged with terrible questions, but you can absolutely help with that once you have just a little bit of rep (downvoting them and/or, if they don't follow the rules of the site, flagging them for closure). (I like taking short breaks at work to clear my head by looking through the front page for such questions - there's almost invariably at least one such.) The existence of crap questions does not make it any less invaluable a resource for *good* questions, though - I remember the awful days when you had to find answers on *shiver* ExpertsExchange. Ugh.