REGISTATION - new user...
I found the image rotation captcha to be a clever innovation i'd not seen before.
That said, it was one of the most difficult challenges i've ever encountered, more so than google's street image challenges (where there are cars, signs, etc. to select the existence of traffic lights/vehicles/signs in a few pixels in the corner of the tiny thumbnails)
I thought i was familiar/conversant with icons and thumbnail images in general, but i was unable to visually parse what one of this registration's CAPTCHA was, and thus at a loss to guess which orientation was wanted.
Adding to the difficulties was that several iterations of trying to "answer" the rotation challenges, several other fields in the web form were not re-populated, and thus posed errors themselves even when i got the captcha correct.
Of course the focus/frustration/feat was for conquering the captcha but not noticing that the password & its retype field and I ACCEPT checkbox were unpopulated (cleared)
-- thus the suggestion is to either alert us that these fields are not repopulated (and we have to re-enter them), or better yet, consistently re-populate them.