The sign up form is good, though it would be neat if you could fit the re-captcha widget more into the style of the site (don't know if they allow that).
The welcome screen has lots of words. Lots of words. I almost went blind. I didn't care what the words said (there were too many) and just started clicking on links, first my silly default profile picture. I would suggest a much simpler welcome UI -- something with an easy-to-follow getting started guide. Numbers help. Large fonts and short strings of text help. Graphics, diagrams and pretty icons help too. I still have no idea what this site does.