Turn on/off Mobile Network
That is a killer feature. These devices are to the point where the phone network is not the only primary function or network connection.
Recently with the iPhone, that feature would have been just the thing I needed to confirm if the phone was using the phone network or wireless signal. At 2$ a meg, it's going to be a big phone bill in absence of a data plan.
Now I have to go see if Maemo 5 can specifically disable the phone network (probably still airplane mode).