I think Nintendo is making a good move by not releasing half-hearted Mario sequels every year. They instead work to create a high quality game. Look at Sonic the Hedgehog. I hate him!! His games are very boring and have no variation between versions. Another example is Tomb Raider. Sure, I love Lara Croft, and I thought Tomb Raider was great!! But I have yet to play Tomb Raider II and I doubt I will play Tomb Raider III anytime soon, as there is just not enough variation between games.
Perhaps the wait between Mario games is too long, but I point to yet another game, Virtual On. This arcade game was 6 years old before a sequel was made, and the original is still very entertaining. I have yet to play the sequel, as it is not yet available in my area, but I still love the original. Mario games are like that also. They are the kind of games that even though they are old, you pick them up because they are fun. I just got a N64 this week, but my sister and cousin still play Super Mario Bros. and Super Mario Bros. 3!!!