The long-defunct VisualHub left a hole in the video-encoding category on the Mac that no app since - even those based on VIsualHub's source code - have been able to fill. That said, Adapter comes very close (even in how it looks and adds a bunch of unique features. It also has the advantage of being completely free.
The app is designed to be easy, so you can choose presets for quick conversion. If you know what you're doing, however, you can just click the "Advanced" button (at the bottom right of the window) and get plenty of other options giving you more - but not complete - control. Regardless of any limitations, there are tons of conversion features for all media types. You can even convert a file you haven't downloaded yet by specifying its URL. If you want a video from YouTube, it'll take care of downloading that as well. Adapter is just a really comprehensive, really fast conversion app.
There is one thing, however, that VisualHub did that pretty much every conversion app has yet to do: create incredibly simple DVDs with practically no effort. While DVDs aren't quite as relevant as they once were, when you need to burn something quick there was nothing simpler and faster than VisualHub. Adapter doesn't offer that feature, much like pretty much everything else in the conversion app space, but it's so great in so many other ways that you're bound to find it useful for the majority of your conversions. And maybe, some day, it'll be able to make DVDs, too. (Hint, hint.)