Everything...... but following things strictly:
1. Appearance / feel of set . I have seen too many refurbished sets in market at hall road and hafiz centre, mainly LG lu6200 and LG f180 (Like all other sony htc refurbs) - LU6200 has dull chrome which is repainted and pathetic shape, does not have shine of chrome. Many F180s are coming in marketing with stickers all around the set and sometimes their casing is changed.. so watch out for such things.
2. Some sets have signal and wifi issue. It might be due to corrupt IMEI or partitions got f**D up due to messing with too many custom roms.. so make sure you get good and continuous signals.. no disconnect.. you need 7-8 hours to be sure it is OK in terms of network signals.
3. General things, Bluetooth, sensors, calling, vibration, sound, screen / touch etc.
P.S. Even if you know many things.... still you have a chance of getting a faulty set.. so LUCK also counts here.. Make sure you get atleast 24 hours replacement warranty from shopkeeper.