Gez said:
A full blown cold vulnerability is overkill IMO. Too much of a drawback.
I like your idea of a -4 penalty on saves vs cold. Id also drop the natural armour, the scales a too small and delicate to provide protection
also you can't have a Cha penalty for a Snakedude - snakes are just too damn sexy

!!!
Serpentkin:
Medium Humanoid (Human, Reptilian)
> +2 DEX
> -2 WIS (Slow thinking)
> +10 Movement speed
> Darkvision 60'
> Cold Vulnerability -4 Penalty on Saves vs Cold
- Poison Immunity Serpentkin are immune to their own venom and gain a +4 bonus to saves vs poison (all poisons)
Feats
1.
Feat:Free Diving
Requirements:Con 11
Characters with this ability can hold there breath for 3 times as long as normal and do not suffer penalties for deep water whilst swiming
2
. Feat Mesmersing Gaze (su)
Requirements:Wis 13, Serpentkin, Must be taken at first level
A few serpentkin are touch with the blood of Basilisk and can mesmerise those who gaze into their eyes. A serpentkin must be able to study a target undistracted for at least a round. A mesmerised target is enthralled by the serpentkin and can take no actions in the following round. The effect is broken if the target is attacked or the serpentkin is distracted
As above
3.
Feat: Poisonous Venom
Requirements: Serpentkin, must be taken at 1st level
Some serpentkin are closer to their roots than others, those that are often retain a snakes poison sacks. A serpentkin with developed poison sacks can secret a poisonous venom, (DC 15 + CON modifier) for 1d4 damage. Serpentkin can coat their hands or weapons before combat. A weapon coated with venom will wear off after a single strike. Also weapons coated will dry up a number of round equal to the CON modifier after the venom is applied. A serpentkin can only apply venom once every 1d4+1 rounds. Serpentkin are immune to their own poison.
A serpentkin can also make a spit attack, range 5' to inflict the target with their poison as a natural attack.
personally I'd make spit attack another feat