I am going to be a bit contrarian: I think that the YUI question is perfectly valid, but the 'any more features' is not. The former is too open ended and non-specific, and is best be answered (in the case of Smugmug, likely not answered) by the company itself.
However, the YUI question is a good one for this forum, because it will impact many who comes to this site for deep answers to complex Customization questions. Since javascript is a critical component of Smugmug customization, I expect, and hope for, much detail Q&A around javascript specifically for Smugmug. API and other issues, like YUI seem like perfect subjects for discussion.
I would expect this question to lead to other answers regarding moving from YUI2 to YUI3 if the need arises.