Pikmin wasn't the greatest game for the GameCube, but it was certainly one of the most original. It showed a freshness both in style and content that's all too rare in games. The sequel keeps with the general practice of leaving untouched what worked while jettisoning what didn't, and since there was little to complain about with the first game, Pikmin 2 is basically more of the same. Whether that's a positive or not depends on how much more Pikmin you want.
In the original game, Olimar had to salvage parts of his rocket ship with the help of red, blue, and yellow Pikmins, small creatures who fought for him and carried items back to his ship. In the sequel, you salvage treasures to pay off a debt, and white and purple Pikmins have now joined your team. Despite the frequent battles, this is a strategy game; you have to carefully utilize your Pikmins, since each group has unique attributes.
There was only one major flaw with the original Pikmin: you had only 30 (in-game) days to complete your mission. The elimination of this in Pikmin 2 was one of the more significant changes the developers made. Strangely, it was for the worse. Turns out the time limit emphasized the strategic nature of the game by forcing you to plan each in-game day carefully, prioritizing your actions for maximum efficiency. (It also gave the game its replay value as you tried to accomplish your mission in fewer days.) With this element gone, the player can accomplish the goals at a leisurely pace, so the sequel is more relaxing, but less exciting.
Pikmin 2 also has a few minor annoyances:
-- One of the most prominent changes is the addition of Louie. With two characters to control, the developers opened up a lot of possibilities where switching back and forth between Olimar and Louie could become essential to solving puzzles or defeating enemies. Unfortunately, you can finish the game without really doing this. Turns out Louis isn't much of an addition to the gameplay.
-- With each treasure you retrieve and each enemy you defeat, an entry is added to your mini-encyclopaedia. But the entries don't enhance the story or give any extra clues. Reading the encyclopaedia is optional, but it would have given the game more depth if all that extra text actually added something concrete to the gaming experience.