You can come up with a throwaway identity on almost any site.
You can, but it's easier to do in practice on some sites than on others. It's not that easy to do here on Slashdot; on Reddit it's almost trivial, and the RES makes it even easier to switch between identities with a couple of mouse clicks. Reddit doesn't require email confirmation and all that crap like (I believe) Slashdot and other sites do, which also causes them to limit you to one identity per email account. Yes, you can just go set up a bunch of one-time-use email accounts, but again that's a PITA, unless you're a professional troll and you have a semi-automated system set up for that.
As for Reddit and its gems/turds ratio, it really depends on the subreddit. They're all different, since they're run by different mods, with different rules, and different groups of people in them. It's not like Slashdot, where there's only one user community, and we pretty much all see the same articles; on Reddit, everyone sees a different version of the site, and sees different articles, depending on which subreddits they're subscribed to. With 250,000 subreddits, there really is not a singular "Reddit community", though there certainly are some subreddits which are extremely popular and might be considered somewhat representative. I can see all kinds of horrible posts in /r/politics, for instance, but then go over to /r/embeddedlinux and see nothing but polite, well-thought out posts (but not too many of them, since it's a small group with only 646 readers).