In Pikmin 1, your ship crashes on an alien planet with an environment that is hostile to you. You can sustain your own life for 30 days, and during that time you must retrieve the parts of your ship so you can escape this inhospitable world. That, sir, is what you call a reason to play.
Pikmin 2... um... the owner of your company needs 10000 credits so he can stay in business. He sends you back to the alien world so you can collect junk. Life support is not an issue, take all the time you need. I lost interest after 6 or 7 "days."
Both games are inherently collectathons with some puzzle solving. But the first game gives you a compelling reason to play, a reason to care. Your life is on the line. The second game? Collect merely for the sake of collecting. Yippee.
That Pikmin comparison is just one example of why gameplay is entirely overrated if there is not a reason for the game to exist in the first place. Give me a reason to play. And, for the love of all that is good, don't make it the very same reason I had the last time around (a la Mario, Zelda, etc.).