In peoples' minds, gentrification is bad.
Most of the arguments against it are poorly supported and/or disingenuous. But the legitimate ones all refer to the instability created by gentrification.
Gentrification has large short term negative effects. Quickly changing neighbourhoods that rapidly rise in value force people who can't afford them to leave. If you're a lower income family barely getting by as it is, you might resent being suddenly displaced from the place you've lived for 30 years. Hell, if you're a lower income family, you might not be able to afford the cost of moving.
The big problem with gentrification is that we lack viable social supports to help people in transition during periods of gentrification. Too many people want to stick their head in the sand and ignore it, or foolishly try to legislate (or activist) it away.