I don't think I would like Ianto nearly as much if he was played by someone else, especially if it had been an actor who just got by on their looks.

I don't think for a second Ianto would be anything like he is now if it weren't for Gareth. You can tell they looked back on season 1 and picked up on things he hinted at in performance and fleshed them out. If they'd picked a lesser actor he'd still be hanging about in the background, retconned all the way back to London, or just given a completely unsatisfying death.