Well, we've had several DMs here who've said that they wouldn't allow a particular feature for various reasons.
There's going to be DMs who've had a bad experience with a certain feature with other groups and have decided to ban the feature altogether (one of the DMs at my table automatically disallows a few feats, including Lucky, for this reason).
There's also going to be DMs who'll reasonably say "I'm going to say no now so we can get back to the game, and we can have the discussion about feature later on" (another of the DMs at my table did this with a particular cantrip, but after the game decided that the players' ideas were sound).