One of the biggest problems I have with Nintendo is how formulaic most of their first party games can be. Take a look at the Zelda series, it's a fine series with many memorable and excellent titles. However, most Zelda games can be reduced to the same basic formula: Green clad boy works to save the Kingdom of Hyrule and the Princess Zelda from some evil (usually some variation of Ganandorf). Over the course of the game, Link will hit all of the following: a Lava area (usually accompanied by the Goron), a Water area (usually accompanied by the Zora), a Forest area, an area where he will need to use "heavy boots" to avoid being blown or knocked away, and every dungeon will have keys that unlock doors in them. He will also obtain the following weapons and items: One boomerang, one bow, one bomb bag, and tons of crystals. In the end, most Zelda games can become boring simply because once you've played one, you've played them all.
The Mario games also can become formulaic. Sometimes this is a good thing (like "New Super Mario Brothers" for the DS, which perfected the formula so well you can easily look past the lack of new concepts in it), other times it can be a bad thing. However, generally speaking, I'm someone who loves to see new things tried in tired series (like Windwaker in the Zelda series, or Metroid Prime in the Metroid series). Which is part of the reason I love Super Mario Galaxy for the Wii so much.