Lee Pace and Mackenzie Davis are amazing in this regard. It's so easy to throw a romance plot into a tech show like this, but it's damn near unique the level of chemistry that these two are managing to put into it. For more than a season their relationship has been in the little looks, and the subtle actions. I'm certainly happy that Joe is back in the picture for season 4.
I'm just about to start a re-watch, so my opinion here might change... But I've never been a fan of Donna. I think she deserved what she got at the end of season 3 but I think it sets her up for more of a villainous role in season 4, which will be interesting because I've always seen that dynamic to her character so now she can kind of embrace it.