Checking it out, I don't see anything in the app that seems to be dependent on 4.0+ features, though obviously things are easier if you can use the latest and greatest APIs.
I'm guessing that it was more of a support issue. Switching to 4.0+ drops the number of devices you have to support dramatically, and with that the headache of all of the obscure device users giving you 1 stars and angry emails.
The team I'm on is transitioning from 2.3+ to 4.0+ support for some technical reasons, but easier development and support is a huge bonus.
