Multimedia support
Scheduling algorithms
What decks are available for download
Cloud syncing
Ease of use (esp. on mobile devices)
etc.

As for "only two sides to a card", in the case of Chinese and Japanese many people want to study characters -> meaning as well as characters -> pronunciation so that's already three sides on a card. ;)

James

Ease of adding and flashcard management - Pleco and Skritter are particularly good here, others involve a lot of extra steps that detract from the experience.

If you are interested in specifics, I think Pleco is good to have for quickly adding characters that you look up during the day. But I also make heavy use of Flashcard Deluxe on my itouch. It requires creating cards through spreadsheets, but I sometimes find that I'm more likely to remember the character when making the cards -- and tailoring the definition -- myself. (And the programmer provides strong customer support for the app.) I haven't experimented with the feature yet, but Pleco does allow exporting the cards that you make.

I'm totally happy with Anki, it has almost all the features I wish.