Only super effective against one type- (Grass). It is the only type that is only super effective against one type, with the exception to the Normal type. Additionally, being good against the grass type is not that impressive. Grass has a boatload of weaknesses to types that generally hit harder (Fire, for example).
Only 4 resistances- (Poison, Fighting, Grass, Bug). Not bad, but not great either. In my experience these are less common types, however.
No immunities. There are 7 types with immunities (nearly half). Poison does not have one.
It does not affect Steel.
Two weaknesses- (Ground, Psychic)- not bad, though these are two common types/moves.
With that being said, there are some good/common/heavily used poison type moves, such as Toxic or toxic spikes, but my main emphasis is on the type itself. Personally, I think the biggest problem is the lack of power poison types. The only type with fewer strengths is the Normal type, but Normal's are common because there are plenty of power pokemon in that type (Snorlax, Blissey, etc) There is a reason that there has only ever been 1 poison gym/elite four user, with none since Gen 2. Most of the best poison types were in gen 1, but they've lost some of their power with many core offensive poison attacks becoming special attacks. Gen 4 was another step in the right direction with Skuntank and Drapion, but then Gen 5 took a dump with only 3 (mediocre) poison types. At the very least, I think Poison should be super-effective against Bug.