So far I like Thresh (support full tank). He's got a nice trick and that's what a good champ needs in my opinion. His trick is the ability to jump into the other team, drop a massive slow, and then have his bruiser (Olaf, Udyr, etc) who otherwise doesn't have a solid gap closer join him in the middle of the fight via the lantern. Very nice.
Getting no innate armor scaling and relying solely on his passive means that unless he ignores the game entirely and focuses only on roaming not to gank but to lick the map clean of souls he gets enough armor to just barely lag behind other tank supports innate armor gain (Leona, Nunu, Voli, etc). This makes his passive kind of pointless and leaves him weak because other champs get the innate armor gain plus useful passives.
So either he needs to have the armor scaling per soul improved or champ deaths need to provide more than one soul or give him a small amount of innate armor scaling or maybe just take out the armor bit from his passive entirely and give him some normal innate scaling armor or something. However you do it he needs that aspect of his design buffed.