A good number of Nintendo games can be beaten in a matter of a few hours during the first time through; difficulty can influence the fun factor, but there isn't always a direct correlation.
Luigi's Mansion is short, and it did really need more gameplay depth. I still thoroughly enjoyed it at the Gamecube's launch, and it's still a game that I'm looking forward to replaying. SGnG, by comparison, was pretty forgettable for me. I rented it once, played it awhile, and found that it was about as interesting to me as a jelly stain on a lampshade. There were much better platformers in the SNES gen.
When it is eventually selected, I intend to add a little more to the discussion than trolling the actual game discussion thread, as I see that you are already busy doing.

Edit: Lindy, I love that avatar. My wife and I saw that episode of
It's Me Or the Dog; hee-larious!