...Replying in a thread that's two days old will hardly give you any XP, no matter how good it was...
Maybe you should only be given XP as a voter when voting on nodes that are at least 24 hours old? Maybe that would encourage looking through the Monastery for old nodes that may be applicable.
Similarly, maybe you should only be able to get XP as a voter on one node in a thread. As soon as you have voted once on a node in a thread, you will never get XP for a vote on another node in that thread.
Liz