It looks like I can finally track my expansion plays without increasing my total number of games played (playing Carcassonne with 5 expansions would only count as one game play and not six).
As a side note, I don't believe that the actual mechanism for tracking plays has been altered. Rather, I believe that many (all?) game-related categories (e.g., owned, commented, wishlist, plays, etc.) have been divided into base games ("board games") and expansions ("board game expansion").
In other words, if you were to log your plays under the base game then they will show up under the base game. If you were to log your plays under the expansion(s) of choice then they will show up under those expansions. I don't see a functionality difference, rather a display difference (which impacts search, too).