1. The form factor - like it or not, the form factor is undoubtedly the most important criteria for people buying a new phone. No one likes an ugly device. This partially explains the success of the iPhone, the amazing form factor.
2. User Interface - abbreviated as UI, this is more or less what makes or breaks a phone. Symbian most say (except for fanboys of course) has the least attractive UI, and android (with sense UI from HTC) and the iOS are generally considered among the best. The others like Maemo and Meego, although not exactly new platforms, but havent been mainstream OSes so far. Nonetheless, UI isnt really their forte.
3. Whats 'inside' - the processor, the RAM, the memory capabilities, etc. This is where android wins hands down more or less. Why? Well, the high end phones have a processing speed of 1GHz and RAM of 512MB. With a GPU (graphics processing unit), thats a killer deal. Sure the iPhone 4 has almost the same specs, but its sort of rendered redundant thanks to its severly restricted multitasking abilities.
4. Apps - applications are what smartphones are made for. And unless there's a plethora of apps to choose from, using a smartphone doesnt make much sense. The android market and the apple app store and legendary in this regard, with Nokia's Ovi Store coming a very distant third.
There you go, the next time you buy a smartphone make sure its a 'good' one and scores high on your GQ levels.
Claimer: these are my opinions so dont expect them to be universally accepted truth.