Clingy Costume: As an astronaut in an experimental sublight journey to Alpha Centauri, he was put inside a copper-lined uniform to survive a thousand-year space journey. If it is ever pierced in any way, he will suddenly age a thousand years.
The One Who Knows: Advised the Guardians on what course of action to take. This wasn’t a superpower, it was Stakar stuck in a time loop where he always relives his life from infancy, knowing what to do from experience.
Deus Exit Machina: Generally regarded as the team's heaviest hitter, Groot was absent from the team's official foundation as he was still recovering from his sacrifice in Annihilation. Likewise, whenever he needs to be written out for drama they simply have him make a slow recovery from a prior battle.
Glass Cannon: So far, he got smashed to pieces in Bendis stories a bit too easily.
Healing Factor / From A Single Splinter: A very impressive one. Groot can, depending on the circumstances, regenerate lost limbs and even his entire body within seconds of being destroyed. Though going from a single splinter usually takes much longer and requires water.