that's my take on it.
I've played all the Metroid prims, and this looks EXACTLY like an upgrade of hunters.
so why all the hate?
I think this all stems from a number of different factors.
1: Metroid fans have been waiting for a new game for 5 (or, if Other M didn't count, 8) years and Nintendo had revealed this instead of a new Metroid Prime game (Although, with the speculations of the next Metroid Prime game probably being made on the NX, this would calm people down a bit and give Nintendo a reason why they did make Federation Force...).
2: You don't play as Samus in the Multiplayer, instead, you play as Federation soldiers. However, with the anouncement of Samus being in the game and, thanks to this evidence, being in a playable state, fans will have to wait until they get it in their hands to see if it is good or not.
3: Blast Ball. What more do I have to say?
Those are just three factors of why this spinoff of the Metroid Prime series is getting much hate.