Hive is the perfect example of such. Beautiful UIs across the board through Hive.blog, Peakd.com, etc.
That, and the games developed here and the user-functionality tools, are also very straightforward and easy to use.

I'm thinking about developing something gaming related and UI is definitely a big part of it. Being too complex or too user un-friendly are definitely factors at play with the success of anything created.

Great post and a good reminder to those looking to develop, that how things look and their ease-of-use will greatly determine their success or failure in their endeavors!