The only thing holding you back is your own imagination.
There's no real story to speak of. You play as Maxwell, a little guy with a funky hat, armed only with a magic notepad that can conjure up anything you can write...well with a few exceptions. You can't write copywrited characters, alcoholic beverages, vulgarities or racial slurs aside from the infamous Sambo melon incident. Still the game can conjure up over 200,000 nouns though in many cases writing several different words will make the same object appear. When you start, you're even given an open area where you can just basically muck around making God and Cthuthlu appear and fight each other.
When you do get tired of just playing around with words, you can start the main game. You have two kinds of scenarios in each level. Puzzle and platforming. Completing each stage gains you a starite as well as ollars which can be used to open up new levels and buy items for the stage builder. In puzzle stages, you are given a clue and have to conjure up what the game wants such as "getting a kitty off a roof" or "catching a butterfly." Where as the platforming stages put a series of obstacles between you and the Starite. The real challenge comes from trying to complete secondary objectives to collect additional Starites. The trick is you do the stage again but use a different method as well as you can increase your score by doing things such as not using weapons, using only animals, etc. It's easy to get into a habit of just conjuring wings to get you around and summoning a Trex to clear out baddies but as I said, trying to do the same mission again without using the same items is where the real challenge comes and having a thesaurus handy doesn't hurt either.
There are some flaws you probably already heard about. First of all, the controls can be a pain. You move Maxwell around by tapping on where you want him to go. I get why they did this. It was not only to make it less of a hassle instead of switching between stylus and d-pad as well as be more accomodating to left handed people but having D-pad controls as an option would've been nice. All too often Maxwell's epilepsy kicks in and he'll run around hopping about like crazy and often you have to start the mission over. Also, at times, objectives aren't always clear and the game doesn't always seem to follow real world logic such as how can an armored car be destroyed with three hits? Why can't a suit of armor protect me more and why do I have to conjure scuba gear if I only have to dive in a small body of water? It gets a bit annoying when the game throws out real world logic.
Still it's really hard for me to complain too much because this is one of those rare cases where the sheer creativity of the game more than makes up for it's flaws and the only thing holding you back is your imagination. Sure there's the easy way to solve the puzzle but I actually found myself wanting to go for a more wacky approach just to rack up more points. And if you get tired of the adventure mode and finish all the levels, it features a robust level editor where you can share levels you created online. I had trouble with it but I'm sure others will come up with something interesting.
Still Scribblenauts will go down as one of the best DS games this year. It's fun and creative.