I thought it was interesting that Luigi was the ideal host - like he's hiding secrets deep down. SSBB also touched upon it with the whole Negative Zone attack and the trophy about "embracing the dark side in his brother's shadow" or whatever. I also feel like maybe he's got insecurities and things that could be seized upon, whereas Bowser, while evil, is confident in himself and feels overshadowed by no one. I dunno, I just appreciate every little scrap of depth Nintendo bothers to give us about their Mario characters.
The main thing that bugged me was that throughout the adventure, Luigi being Mr. L was treated like a joke: imho, it would have been a lot more interesting if Mario knew all along that his twin brother had been brainwashed and was being used as a weapon against the other heroes - giving him a personal reason to be going after Bleck: more Yoshi's Island-ish "gotta save my other half" quest-ish, instead of just the usual "gotta do what's right, gotta save the world, impersonal and flat do-goodery, lah dee dah" routine.
