I just played against an AP Master Yi on dominion. and for the most part hes fine, except for his meditate. He can meditate under a turret and go full hp while taking absolutely no dmg for several seconds. His cool down on meditate is also way too short, every 20 esconds or less his meditate was up. I believe this makes it very unfair for the other players.

but he can't do anything while meditating...

Get a stun, a silence, a pull, a knockup/knock-back...?

^ And if you don't have those, you can buy an executioner's or Morello's, hit him with the active and massively reduce his healing.

AP Yi using meditate is far more effective than it should be.

That said, just get someone with one of the aforementioned CC abilities against him and he becomes useless. And I mean that, useless.
I don't find Greivous Wounds to be enough to counter Meditate, he's still stalling with +hundreds of resistances and still healing a LOT. CC is the way to go.

Riven says hi.

All I know is Soraka makes him her *****...silence and he runs from the horrid MRes shredding

I was already Ryze, I still beat him, but that meditate is annoying

His meditate is incredibly strong... but, well, that's all he really does. Sure it's super annoying if you don't have an interrupt (Exec Calling doesn't really help as much as it should), but that's only relevant for bot lane. Worst case scenario; you stalemate and can't pressure him off his point. Oh well, too bad.

If he's top and you don't have any interrupts... that's a problem with your team, not AP Yi.

With the Crystal Scar aura, grievous wounds brings him down to only healing 30% of his meditate base, it's still a considerable downgrade.