well, I am not disappointed and was not at all disappointed while watching the show.
I pretty much think that he's himself, and much more than just a replacement for Mats - I truly hope this was everybody's intention as well.
Maybe we just got different impressions from watching the show on different "media" so to speak - online and on spot; maybe it's also a result of the emotional background while watching his first performance with Therion and of the expectations we had from him.
As far as I could hear, Thomas' voice is cleaner and more "round" than Mats', at least in those songs that are Mats-"branded" where you could really try to compare them. It could be also that he has an excellent control of his voice in general, including the range involved in those songs I'm talking above.
As I said, I also think - from the short set I could hear at Wacken - that Thomas' voice is extremely versatile and somehow I heard there, at Wacken, a promise that this will make possible for new songs to be performed on stage...
However, by saying this I don't want to end up in a comparison saying that X is better than Y, 'cause both of them are outstanding voices, but I think each with its own identity.