you should only have 1 piece in your group by clause, and it should be r.repTopic.
But honestly I've seen better results using something like this:
t.ID AS tID,
t.topTitle AS tTitle,
t.topDescription AS tDesc,
t.topCategory AS tCat,
(SELECT COUNT(repTopic) from replies WHERE repTopic = t.ID) AS tPosts
Only because when you do the count, you don't really get a good benefit from the join. Let me know if either of those work.