I'm actually surprised to find that Luigi has a lot MORE strong points. He can in fact jump higher than Mario - and slow his descent like Yoshi can. AND he's taller and more athletic than Mario. And he's also Stronger too.
The only handicap that I see is that he's a bit of a Coward and a little too shy. Instead of barging into Danger Head-First like his brother, he's more than willing to hang back and let Mario take point.