The only things GTA V had going for it were better graphics, a livelier Los Santos, golf and Michael's bullet time. GTA: San Andreas had more cities with varied environments and small towns, more memorable missions and characters and a better overall licensed soundtrack. You could also crouch/sneak in GTA: San Andreas but not in GTA V (only in cover or a cutscene).
The tweaked Euphoria physics in GTA V are also a step down. You can't even pick up litter or miscellaneous items like you could in GTA IV to throw at people. You fall down in GTA V and take longer to get back up. Franklin is the only decent protagonist in GTA V, though not as memorable as his friend Lamar, and not as great as Niko from GTA IV, while Michael and Trevor are simply annoying. You can no longer enter hospitals or order fast food like in previous games. Remember the black Burger Shot employee who quipped about Niko having an iron stomach? He was funny. I like to think he and Niko became friends.
Previous games in the series also had heists, starting with Vice City. The only real fascinating thing about GTA V was being able to switch between multiple protagonists, an idea Leslie Benzies planned for GTA: San Andreas but scrapped as it wasn't technically viable.
Now I don't think GTA: San Andreas is the best in the series but it is a lot better than GTA V as an open world playground with a lot to do and see. I think most people who dislike it just never got far enough in the story to appreciate its variety, and characters like Woozie, Mike Toreno, and even Ken Rosenberg and Salvatore Leone. It was a nice bookend to the 3D era. GTA V only had a passing mention of Niko by Lester and Packie, which was kind of a cool easter egg at the time, but Trevor kills Johnny for some stupid reason. Although to Rockstar's credit at least it was Johnny and not Niko or Luis.
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