Pretty much everything, really. Prime and Prime 2 have awful story/backstory/canonical relevance. Other M has a great story, a good backstory and actually has relevance to the Metroid canon. It's also got much better voice acting than the other two, not to mention characters. Prime 1 and 2 pretty much just have Samus and Dark Samus, which is... well... not a lot to work with. Other M also trumps it in user-friendliness. I know lots of people who dropped Prime 1 because of how incredibly hard it is to navigate. Other M fixed this with a handy map.
Really don't get the hype for the Prime series. Other M is so much better in pretty much every respect. Thought people would realise it was just overrated once Retro revealed themselves as awful developers with DKCR. Waggle to roll? Really? The same people who made the 'amazing' Prime games?
Really don't get the hype for the Prime series. Other M is so much better in pretty much every respect. Thought people would realise it was just overrated once Retro revealed themselves as awful developers with DKCR. Waggle to roll? Really? The same people who made the 'amazing' Prime games?