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Torterra has mediocre abilities, an unfavourable typing, and a weird stat "balanced" stat distribution.
He has decent defenses, and a useful resistance to edgequake, but only an electric immunity beyond that. When combined with a 4x weakness to ice and an average special defense at best, he isn't that great of a wall.
On the offensive side, he has a good base attack for lower tiers, but his attack is pretty underwhelming when unboosted in lower tiers. He does have access to edgequake, and gets STAB from eq, but his grass STAB doesn't add much to that.
Really, the best thing about torterra is his access to leech seed (and immunity to leech seed as a tank), synthesis, and his low speed which he can use to abuse trick room.
He has a unique typing, but I think many other things could do his job better in OU.