Just to ditto what everyone else has said, the research capabilities are phenomenal. I don't think I'll ever have to google a pattern or a yarn again! It's like a one-stop shop. And the possibilities for searching just seem to be multiplying all the time, as new features and new users are added. Search by pattern, by yarn, by designer, by popularity. Clicks on "Friends' Activity" and see what all your friends have been up to (what they've cast on, what they've added to their queue, what yarn they've stashed). The groups/forums are adding another dimension as well.
Here's a for instance on how cool the search stuff is. On vacation, I bought some Handmaiden Sea Silk simply b/c it called my name. Didn't know what I was going to do with it. Got on Ravelry. Searched under yarn for Sea Silk and got to look at every single project that anyone there was doing with that yarn. I saw a few shawls I liked, for instance the Gothic Leaf Shawl. It told me how many people were doing that particular shawl, and I could look at every one of those projects (those in Sea Silk, those not). See how people rated the pattern in terms of both difficulty and overall satisfaction. Could read people's notes about the pattern. Could follow links to other patterns by the same designer. Found a Gothic Leaf Shawl someone was working on that I particularly liked. Clicked on "Add to Favorites" so I won't lose track of it (now it's in my notebook). Clicked on "Comment" and sent a question to the knitter about it. After she answered my questions, then I clicked on "Add to Queue" and it asked me things like what yarn I was going to use, what needles, when I wanted to finish it. Then when I was ready to cast on, I went to my cue and clicked "Cast On." I didn't have to add any new information about yarn and needles, b/c it was all there.
That's just one small "for instance." Multiply that by many possible searches, and you can see why it's such a time suck!
Add to that the ability to catalog and organize your own stash, projects, and plans, and it is just completely amazing.
And the more people who get added, the better it gets!