It's unfortunate to lose Thomas. Though he wasn't quite as consistent or good in his second season as he was in his first partial-season with the Rapids (and he earned a somewhat overblown reputation for dirty play at a few points as well), he was still a precision passer and one of the most important hubs for possession on the entire team, as well as a rock in the center that allowed Nathan Sturgis to be a bit more free-running going forward. Obviously, he chipped in a couple of goals as well.
Thomas had been in the middle of a contract issue, as he was reportedly asking for Designated Player-type cash and the Rapids weren't biting. Thomas made a cool $200,000 in guaranteed compensation last season, and will probably get his salary bumped up in Dallas, a team that weren't facing the same type of cap crunch that the Rapids were going into this off-season. Said cap crunch should be alleviated a bit now with Thomas off the books.
Jose Mari was brought into the preseason likely to act as Thomas' replacement in the future, so it won't be a surprise he's given a full contract some time soon.