To encourage more discussion and allow users to easily track the ideas they're interested in, it would be helpful to enhance the current activity stream on a user's profile or otherwise direct users to updates from within IdeaScale (without bouncing between it and email to follow updates). I think there's a relatively simple way and a more complex way of implementing this solution.
1) Simple - Since all events (votes, comments, idea submissions) are tracked already, the addition of a 'more' link or some other form of pagination would let users trace more of their activity if they're looking to revisit something.
2) Complex - List (by recent activity) all ideas that a user has interacted with in some way, so they can follow any new activity updates without going to email. For even greater detail, add filters/tabs for Ideas: a) submitted; b) voted on; and c) commented on.
Another possible enhancement would be an 'alert' system that prompts users to visit ideas they've participated in that have been updated since their last session/page refresh/etc.