The problem with a lot of black people, in this case black men, is that we have been the medias laughing stock for so long that any thing that could perceived as a slight is worthy of ire.
If you're not writing a black character that plays up the characters intelligence, strength, etc. then you're not taking a positive step to undo how we're perceived.
Finn is interesting. Here is a black character who will be in the biggest franchise the world has known. He was marketed as a god damn Jedi. We already had S.L. Jackson play a jedi, but not a lead one, right next to a white chick doing the leading for once.
I guess a lot of black men saw it as a vessel to present to the world an image of our might; who was strong, could lead, and could get the white girl. On world-stage!
When the character could not live up to that all hell broke lose. If Finn was played by Zac Efron i bet the character wouldn't receive so much criticism. We're just not used to a lead black fantasy character either not being hyper-masculine, or playing the comedic side-kick role.
Objectively, he was a typical young, inexperienced, but nevertheless brave character. He will clearly step up. But because he couldn't be everything we fantasise him to be, he's useless in the task of undoing that old image that we feel the world has of us.