I find that quite a bit of people couldnt get into the game. I think the Prime subseries is the best series to be released in the last 10 years, however, the very purpose of a Metroid game is a nonlinear exploration based game. You do what you want to, how you want to, and when. For a lot of people it could be too much thinking. Id say that most that dont like the games simply dont want, or cant cope with the mental challenge.
Of course, some people simply dont like the slower pace of the Metroid series (excluding Corruption).










