The fans who don't like Moonraker object that it's literally and figuratively "not on this planet". I think that's a fair enough reason to not like it. The films I can't figure out why others really hate is QoS and TWINE.
That's a pretty weak reason considering only the final battle takes is set in space. Even then, being aboard the space station is not all that different from being inside a volcano base or a super tanker. Just a little less gravity. 
Anyway, those fans should make a distinction that Moonraker is their "least favorite" Bond film, because it's certainly not the worst.
Even if MR came out in 1981 or even 1983 it would have still been released in the wake of the Star Wars sequels, so I'm not sure why it gets so much flak for being pushed ahead of FYEO. It was bound to happen at some point. Bond has been flirting with space travel since Connery tried to board the rocket in YOLT. Do you think Lewis Gilbert's MR would have been more faithful to the Fleming novel if Star Wars didn't exist? I kinda doubt it.
Hating on QoS and TWINE = total mystery to me as well