It would be cool if there was an option to filter for decks that have placed in the top x at competitions, and maybe even the option to search for decks based on card text (for example to search for infect decks)... also, just a heads up, but the "Containing cards" deck filter option doesn't work right on Firefox 4 beta6.

Regarding the top decks in the events, for each game you can see the top 3 decks here: http://deckbox.org/games/mtg/events (for MtG), but true, there is no filter in the decks page for this :)

Yup, and those are only the most recent ones...