As a single player 'just get to the end of the game' thing, yeah it's easy. You can effectively go through the core single player experience with a single well balanced team. If you want to play it like that, that's fine. $30 and 20 hours well spent.
There's a lot more to Pokemon than that.
There's the whole 'Catch 'em all thing' that they've tagged onto it from the beginning. Each new game adds around 100 more. Diamond/Pearl has 493 Pokemon--and then Male/Female variants of 95% of them. Sure, that's not for everyone, but a big draw originally of Pokemon was the Collection aspect of it.
Along with this, there has been Pokemon Breeding since the GBC days. Besides making a copy of a Pokemon you already have, breeding allows for unique traits and learned abilities that the Pokemon would not normally have. There is an incredible amount of depth to creating a specialized Pokemon through breeding that goes far beyond pairing 2 opposite gendered Pokemon.
Then there are the contests. Again, these are not for everyone. In fact I barely even touched them in previous games and regarded it as a throwaway extra. However in D/P, contests are actually a lot more involved. Previously, all you could do was use certain attacks at certain times to win over the judges in a few categories, and that was pretty much it. Now, there are 3 stages to the contest.
Note, this is from the one contest that I played (and failed at) in this game.
First, you 'decorate' your Pokemon with various things that come up on the touchscreen: clouds, bows, sparkles, hats, stars, w/e. Perhaps you can purchase/obtain more and varied items through game events. That part is set to a time limit, and you are probably graded on matching stuff and organization.
Second, you make your Pokemon dance. This goes into a rhythm game similar to DDR, stuff comes on screen and you have to press the appropriate button at the appropriate time with the music. This is where I failed horribly, as my Japanese reading isn't very hot and I didn't quite understand how it worked. But I was surprised and delighted to see something like this in the game.
Last, is the tried and true 'do attacks to impress the judges' stage.
In previous games, you could use Pokeblocks or something to improve certain stats for these contests. There's probably something like that in this game. I've seen odd machines that are probably the mixers, but haven't tried to use them.
Linked to the Pokeblocks, there are several spots around the game where you can pick/plant/grow berry trees. So if you like gardening, Pokemon has your fix
Then there is the Underground. Every area in the game has a corresponding Underground area that you can access as long as you are standing on the ground. This means, as big as the land on top is, there's about as much space underground for you to tinker with. This is where you can excavate stuff (including Pokemon fossils and the Evolution stones) through a pretty nifty minigame, set up your secret base, and likely do some other stuff.
These are all things I encountered with my 2 weeks of playing the game. With the huge language barrier it presented and my short time with it, there's likely a few things I have not seen and don't know about.
All of that is just the stuff for *single player*, not even touching multplayer/online once. If you do step into multiplayer (particularly online), the value expands infinitely.