From a review blog:
Here's a theory - DMC4's game world feels like it was built from the ground up for Nero, with bosses and level design tailored to his Devil Bringer attacks. Perhaps Capcom, at some point through the game's development simply thought they couldn't release it without having fan favourite Dante playable at some point. And so, rather than redesign the game, they had our stubbly anti-hero thrown into environments and facing enemies that weren't designed to get the most out of him.
It's a crying shame, because with a little more effort, DMC4 could have been a genuine contender for 2008 game of the year. As it is it's a brilliant, addictive, just plain cool hack and slash action game which goes some way to bringing the series back to its initial lustre. There are some high quality contenders to the throne - the God of War and Ninja Gaiden series' spring to mind - but there's something uniquely appealing about DMC. Capcom has done nothing to make it more appealing for those who have passed on the series in the past, has been very brave with its decision to replace Dante with Nero, and, in many areas, been extremely slack in terms of game design, but, on the whole, DMC4 will leave fans with a smirk the great demon hunter himself would be proud of.
They basically say that if Dante wasn't even in the game, and they just focused on a Nero a bit more the game would easily be a contender for GOTY 2008. (at least for action genre).