1. 1) TimelineLite & TimelineMax - how does the API "feel"? - Does it give you the power and flexibility you need without being bloated? Have you found yourself needing some kind of functionality that's missing? Is everything intuitive?
2. 2) Should ALL plugins be activated in TweenMax by default? - I haven't done so because I'm afraid it may be seen as "bloated" with the extra kb. On the other hand, it's not terribly difficult to open the class file and prevent activation of certain plugins if you need to conserve file size, and activating all plugins would prevent some potential confusion for newbies. Keep in mind that there will likely be more and more plugins over time that offer added functionality, so if they're all activated, TweenMax may hit 30kb at some point in the distant future.
3. 3) Should I eliminate the default plugin activations in TweenLite? - This would cut the default file size by around 40%. When I asked this question in v10, I got a fair amount of feedback saying backwards compatibility was much more important than small file size, especially because end users could delete the activation code inside the class file if they needed smaller file size. Then again, TweenLite is supposed to prioritize small file size and a lot of end users are either scared to or don't understand how to delete the activation code in the class file.
4. 4) Are there any important features missing? - I'm trying to prevent bloat while at the same time provide tremendous flexibility, power, and speed. It's a balancing act. Let me know if you were really hoping a particular feature would make its way into one of the classes.
5. 5) What is your favorite feature (or features) that absolutely, positively shouldn't get cut? - I'm not fishing for compliments here - it's just helpful to know what the community values so that those features don't end up on the cutting room floor (so to speak).
6. 6) Should "repeat" be renamed "cycles"? - "repeat" seems more common, but some have suggested that "cycles" is more intuitive. If it gets changed to "cycles", what would you name "repeatDelay"?
7. 7) Should append() and prepend() be eliminated from the timeline classes? - One developer suggested eliminating append() and prepend() because they're not absolutely necessary. For example, append() could be accomplished with myTimeline.insert(myTween, myTimeline.duration). prepend() is more difficult, though. Personally, I find them very intuitive for building sequences and they don't cost much kb.

If you haven't tried v11 yet, especially the TimelineLite and TimelineMax classes, I'd highly recommend it.

By far the biggest news in terms of functionality is the addition of the "TimelineLite" and "TimelineMax" classes. They make building and managing sequences/groups of tweens simple. They originated from TweenGroup, blossoming into something much more powerful, flexible, and intuitive. Think of the timeline classes much like MovieClip timelines in the Flash IDE where you position individual tweens over the course of time.