I like when Nintendo tries mix things up and try to innovate people complain, but they also complain when they do the exact same gameplay over and over again. They just cannot win.
I don't think it's necessarily the same people complaining about both situations. And change is not universally good, it is only potentially good. Metroid remains highly praised as the series has innovated but has also maintained a high quality. Star Fox has changed a lot as well but the changes SUCKED so Star Fox has lost its prestige.
Neither Super Mario Sunshine or Super Mario Galaxy are cookie cutter sequels to Super Mario 64 and yet one is almost unamiously praised while with the other opinions are split. Both tried new ideas with Mario but I say the basic sum-up is that the waterpack was a lame idea and the planets was a good one.
I think a balance of fear and excitement is a normal reaction to an innovative sequel. Hell, I'd argue if this reaction doesn't occur then Nintendo isn't making a very interesting sequel. You're excited to see what the end result can be but are afraid that they'll **** it up. And sometimes even the most talented companies miss the point and turn a beloved franchise into junk. Hate to pick on Star Fox but there's an example of Nintendo fucking it up.
I personally don't have the same trust of Nintendo as I did in the past. Their interest in non-gamers and their insistence on forcing poor controls down our throats for the purposes of marketing to said non-gamers has created a situation where NO Nintendo game is a sure thing for me anymore. Except ironically with a cookie cutter sequel like Super Mario Galaxy 2 which I know will be decent because the first game was. They've become inconsistent, at least regarding my own personal tastes.
If Nintendo has set out to make a great Metroid game then there is no worry. But if Nintendo has set out to make a Metroid game that is successful with the Japanese market that has not traditionally embraced Metroid then there is worry. The difference in approach can make or break the game and this gen we know Nintendo will go in with a purely market-driven attitude.