i) Most Compatible with other device
ii) Most of apps and games are free available.
iii) It can be legally unlocked, I have done it ( T-Mobile gave me code) so the updates are done even it's unlocked.
iv) Physical keyboard as well touchscreen.
v) Open source plateform, you can develop and install on it ( this should be first excitment for techy only other wouln't value ;)
Apart from all this I must mention, T-Mobile customer service is Excellent !!