It's not short for a handheld game if you're determined to 100% it ... you have to beat all levels (regular and special), with both M and L, get all star coins, and get a gold flag on all levels (1up jump on the flagpole) .... to unlock the final level. Beat that and then you can say you "beat the game."
I'm about halfway there. Have to go back and do it all over as Luigi now. Got the game for Christmas but went nuts on MK7 online for over 2 months before I finally picked up 3D Land. I think they made this thing perfectly ... those first 8 worlds are for the little kids and lightweights so they can be happy they "beat the game" ... the special worlds are a mixed bag. Some are easy, while some are genuinely challenging, but still fun not frustrating. (I have to admit, my favorite game of all time is Yoshi's Island, and while I've beaten it twice on SNES and once on GBA, it definitely does get frustrating at times.)
Squeeze every drop out of the game and you'll get your money's worth.