On thing I'm finding to be a huge limitation for us is the fact that currently Sencha Animator is essentially only useful for replacing the type of Flash animations we don't typically create anymore. You can add lines, circles, images and text (all essentially are divs in Sencha Animator), animate them and have Sencha spit out the html/css for you to include in your own pages/apps/posts/sites/whatever.
A more realistic (and useful) use case for us would be to be able to bring in existing (semantically meaningful) HTML docs (with H1, H2, p, li, etc elements) where we would then be able to use Sencha to define animations and transitions on defined elements (either by .class or #id) within the document. I can't help but feel that at this time Sencha is missing a bigger opportunity to define how motion (and interactivity) is defined on the web, rethinking many of the old (and essentially useless) metaphors (timelines, stage, etc) in the process.
As Flash CS5 (and no doubt other Adobe products) will be re-tooled over the coming months/years to produce 'HTML5' animations (via canvas currently, CSS3 eventually) I'm not sure there's much that Sencha Animator currently does significantly better than the more familiar and industry standard tools.