Wayland's goal is to replace the X server on Linux with the Wayland system for a graphical windowing environment. That replacement concept is a big part of the 'hate'. Those of us that use the features and/or like the potential of X do not want to see it go away, and so resent the threat of getting it taken away. As far as FOSS being about choice, that's fine, but if Wayland pushes to get rid of choice, then that's a bit different. Wayland wants most if not all Linux apps to be written for Wayland eventually. Where's the choice there?
As far as the rest of the hate, a big part of that comes from Wayland implementing , from a technical standpoint, the kind of windowing environment that a lot of technical people know to be inferior to what we already have. I don't care how good the code is, if the code is doing something that's not worth doing, it's still a bad idea. same with the ease of writing for the new system: I don't care if it's easy as can be and super clean to write apps for it. If the window system isn't doing he right things, then not going to show any kind of support for it. "Hello World" written for the command line is easy and cleaner than a GUI, doesn't make it the best way to write apps.