I thought Selma was really good, better than 12 Years a Slave in my opinion. Although I don't really rank 12 Years a Slave that high, I thought Dallas Buyers Club and Wolf of Wall Street were better films last year.
I thought Selma was a good movie and deserved to be nominated, but I don't think it should win best picture. I've only seen The Imitation Game, American Sniper and Selma this year so I haven't seen all the nominated movies yet, but so far I would rank the movies like this: The Imitation Game>>> Selma>>>>>>> American Sniper.
I think most people are up in arms about the Oscar noms becuase they feel there is not enough diversification and so they wish Selma had more noms to diversify the nomination pools. Atleast that's what most people were complaining about on Twitter. Oyelowo probably should have been nominated, but I don't think its that big of a deal personally.