Mine would be
1) Transformation - sub section would be - number of alt modes. as long as it isnt forced looking or senseless altmode - I'm looking at Rid Galvatron right now... a giant hand - oh really now?
2) Articulation - the more the better, plus ankle tilts for the win!!
3) Combinations and partnering - Gestalt, Or trailer combines to make armour, that sort of thing. With Partnering - e.g. Soundwave and his Cassettes - now who didn't get entranced the first time you saw and heard - Ejct! Eject! Eject! That kind of Partnering is as Iconic as can be. Too bad though that the gimmick worked only great with Players and their cassettes and not for other TF characters.
4) Unique character Weaponry - each character is given a specific weapon - well except now for the cyberverse which rehashes the weapons. Some weapons have become so iconic and are well associated to the character owning it themselves - e.g. Megatron's Fusion cannon.
5) Light piping - particularly not just limited to the eyes but also for those using light up gimmicks for weapons - e.g. MP Grimlocks weapons
6) Binary Bonding - especially the updated versions - e.g. RtS Cyclonus with Nightstick - it's like you get 2 bots for the price of one...
7) The standardized 5 mm posts which will fit any weapon be it from small character to big one and even old weapons to new toys.
8) Stickers - call me old school , but I like putting up the stickers on the toys. Granted to days toys have molded details into the plastic which is great, but stickers brings out a fresh crispness to even an old toy. 
9) Chrome parts - when done right and not in excess - it makes a beautiful toy even more desirable - e.g. Henkei Astrotrain.
10) Minicons - I love my minicons!! Better than keys as these little buggers are cute as too, IMO. Also, they come in such wide array of altmodes ( granted they get recoloured into different characters all the time but, they are a cool gimmick IMO.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]