Yes, it annoys me!
Nope, I'm fine with it.
However I do it too when I'm alone and playing guitar or even listening to music sometimes. It looks ridiculous and it annoys me, but I understand why they do it and how awesome it feels lol
This is another YouTube comments-inspired poll. Personally, while I don't think pronounced expressions and body movements lend to emotional depth, they don't undermine it, either. Showmanship and craftsmanship are not always mutually exclusive, although too much showmanship will certainly direct attention away from craftsmanship. Personally, I'm happy to see more showmanship in a Hungarian Rhapsody encore - Liszt and Paganini certainly did not lack it. Of course, if making faces is all you can do while playing a Chopin ballade or Beethoven sonata, then the musician is no more than a machine cranking out sounds and facial expressions.
Their lack of composure often leaves me feeling a little embarrassed for them as they can look a bit daft.
It doesn't annoy me, but it doesn't impress me either; grand theatrical gestures in performance are superfluous. In my experience working with numerous professional classical musicians in the recording studio, I notice that during an extremely technical or emotional passage, theatrics disappear. In fact, performers usually become stern and stone faced during extreme periods of concentration. In fact, in the studio you will see none of that nonsense. Theatrics during a performance is an unnecessary parlor trick.