I find that I enjoy games more when they surprise me. They can surprise me in terms of story (Metal Gear Anything, I played MGS3 with intentionally avoiding spoilers, it was well worth it, I didn't do that for MGS2), or in terms of fun gameplay mechanics (Mario Galaxy, Katamari Damacy), or just by being a better game than I expected (Zack and Wiki, Okami, World of Goo). The surprises are in different ways, so having them spoiled comes in different ways.
For a very story heavy game, like a Final Fantasy or Metal Gear, the plot twists are what you should hide. Gameplay mechanics, I'm fine with.
For a game where you're supposed to be constantly impressed by the gameplay mechanics (say...Zelda), you shouldn't spoil everything you can do in the game. It should suffice in the review to say that the reviewer was impressed with what mechanics are available.
For a puzzle game, obviously, don't spoil the puzzles.
And for any game, don't hype it beyond what it is. Sometimes I'll play a game after hearing a tremendous amount of hype about it, and I'll be disappointed, not because it was a bad game, but because the pleasant surprise of how good the game is is ruined.
As a general rule to the reviewer, consider what about the game you enjoyed because it surprised you. If this occurs more than a few hours into the game, and it isn't critical in making a decision to purchase the game, leave it out of the review. Let the player be pleasantly surprised. If you want to say something like "Mario saves the princess", that's ok. We knew that was coming anyway.