This is how sequels are done!
Instead of collecting ship parts though, you collect treasure. What does this mean? The time limit is gone. You are now allowed to roam the environments as you wish. And when you go underground, no time will pass. The challenge from the underground levels come with protecting your pikmin. Not to mention, this is the only place where you can get the new pikmin, purple and while. They are transformed states of your regular pikmin, which makes sense because they're powerful. The multiplayer is also a worthy addition, but isn't the main attraction. Last but not least, you can control 2 commanders, Olimar and Louie, which allow you to split your forces and multitask better. Not that you need to multitask, but the option is a plus.
Pikmin 2 improved everything that was great about Pikmin 1, and fixed all the problems the first one had. It also adds a new dimension to the game with it's underground levels, varied environments, all new challenges and power-ups, and... wow. Take notes game developers, this is how you do a sequel.