This will be like beating on a dead horse or something since Blizzard keeps making posts about it, but Aratil put up a comprehensive FAQ about the new and mysterious Arena matchmaking system. This is probably the most enlightening post of all, and should clear up many questions players have about their ratings versus their performance. It explains why some teams are experiencing drops in personal and team ratings even when they sport winning records.
In a nutshell, the system is continuously trying to place players and teams in the bracket. This means a 50% win ratio, where teams are fighting other teams of equal skill (and thus have an even chance to win or lose). If players get winning records but still drop in ratings, it means that they are beating far inferior teams and the system will adjust their Matchmaking Ratings or MMR to the appropriate level. It is by far the best post about the subject, answering important concerns clearly and without any smoke and mirrors. The way Aratil explains it shows how the new system is superior to the old one. If you're going to read just one post about the new system (and there's a lot), this should be it.