Dear Bart,
I would really much like it if you were able to add a feature in which you would be able to see on which collections you collaborate, preferably in the "My Collections" section. I hope this makes sense, I have also asked this in IRC, so probably you know what I am talking about. If not, you could try right now to go to Collect > My Collections, and you will see that the collections that you were invited to as a collaborator, won't show up. It would be really convenient if it did.
Second, a request by Redshrike (to keep forum pollution at a bare minimum) and I agree on this one, is that it would be really convenient if you could follow comments on collections, with a notification in the mailbox.
Furthermore, keep up the good work, that is my last request, I promise! ;).
I'll get to this when I can. Thanks for posting this. :)
A related request: It would be nice to have one's favourite collections listed somewhere. After all, giving feedback is only one part of marking something as a favourite; another one is making it easier to retrieve (for oneself). At the moment, my favourite collections show up neither under "Favourites" nor under "Collections" in my profile.
I was also going to suggest that art entries are accompanied by a list of collections they are in (to help finding similar art). I see it is the case already - but only if I am logged in. Why is this?
Spontaneous idea: How about "People who marked this art as favourite also marked..."?