As well as the 8 normal worlds - there are 8 Special worlds for you to work your way through that are incredibly challenging and Mario experts or veterans will consider a welcome challenge. As an added bonus, once you complete the requirement for it, a secret will be unlocked. Speaking of secrets - there are a TON of them. Little throwbacks here and there to past Mario games (The timing of the flagpole, the warpzone, and so on). Longtime fans of the games in the Super Mario series will be pleased.
One last thing - the 3D feature in this game is excellent. Depending on your hardware you may experience otherwise (I've tried it with a 3DS XL and New 3DS XL) but the game looks amazing and the levels truly come to life in 3D mode. It's something that has to be seen for yourself to truly appreciate - but take my word for it... the 3D in 3D Land (How appropriate) is better than most games on the handheld.
If you're going to get one Mario game or even just one game for the 3DS - this should be it. It has everything a fan wants - Excellent platforming, secrets, nostalgia, varied bossfights, every power-up you could want and collectibles for added challenge. I highly recommend this for any first time 3DS owner and 3DS owner in general. If you don't have this yet, get it now. I promise you won't be disappointed.