You'd have to be pretty stupid to confuse difficulty for gameplay. Some of the best games I've played have been overwhelmingly easy, such as all of Humongous Entertainment's adventure games (putt putt, freddi fish, pajama sam) and Starfy. On the other hand, some of the hardest games I've played have been terrible, such as the NES versions of Dragon's Lair and Rambo.
Besides which, the puzzles are left intact no matter what difficulty you choose, it only affects enemies. Also worth noting is that MP2 is hard even in normal mode.
Of course, most people who think that all good games have to be hard are the same pretentious jerks that call themselves "high level gamers" and think Street Fighter 2 is the ONLY good fighter and refuse to play games like Smash Bros without dumbing them down to no items/final destination.
Popcap makes casual games. You can hardly compare Metroid Prime on normal difficulty with Bejeweled.