1) IT WORKS! -- Unfortunately not a given - i.e. it does not crash, runs apps reliably, picks up signals, oes not break easily, etc.
2) Form Factor -- Size, display quality within size, etc.
3) Battery Life -- Goes to 1. If the battery is dead, it does not work.
In order of priority:
1) Size, meaning not too big, not too small. Must fit in front pants pocket.
2) Battery life
3) Performance -- fast processor and fast enough internet connection.
4) Weight...because it travels in your front pants pocket.
5) Ruggedness. Gorilla glass and a neoprene skin usually takes care of this.
6) Interoperability with other devices in my CE ecosystem -- share or display media between phone, tablet, TV and desktop PC.
7) Thinness. Sure, why not? This usually goes along with light weight anyway.