I think the most important criteria - given that they are all professional dancers - is that they seem to enjoy being there. Because, if a pro is just going through the motions, their partners and the audience will feel that and they won't enjoy the experience or the show as much.
Since the pros don't get to choose the music (or, I understand, the theme) anymore - any pretence of them having much control over the content of the dance is largely that - a pretence. So, if a pro is weak on a particular dance, they should bring in someone else to help them in training or with choreography. After all, we're supposedly voting for the celebs anyway - and it's not like they're doing the choreography!
So - that being the case - I think that Janette and Anton both deserve to be in the line-up and work, because they both seem enthusiastic and like they're having fun.
The only pros who strike me as if they aren't having fun are Brendan and Ola - and, to a lesser degree Aliona - but that might well just be her manner, so she doesn't seem unhappy to be there, just not actively 'happy'!
I am not sure about Gleb or Giovanni yet, I think they are still finding their feet, so we won't really be able to tell if they're enjoying it properly until they have a second season.
I would love to see Joanne and Iveta brought back into the line-up.