Granary is fine as a map. It's different to all other maps in the pool for a few reasons:
Soldier is the strongest class.
7 cp playstyle
Tight chokes make it play differently, easy to learn holds/stalemates.
Easy to push out of last if you win a fight on last as defenders, as well as easy to push to last if you win a fight on second.
Lots more flank potential than other maps.
Mid and last plays like an arena fight testing team coordination over strats.
Plenty of offclasses easily viable.
Very basic strategies like ubertrading very viable.
It is such a unique map, I never get why the people who complain about it are the same people who want a new, fresh map. Look at process and sunshine, nearly identical maps. Look at metalworks, the "worst" parts of granary are so much more apparent there, but people still prefer it for some reason.
The problem with granary is that people don't know how to play it, not that it's a bad map (same with product and badlands). Granary and badlands are the two maps that people knew how to play better 3-5 years ago than they do now. If you want to learn how to play it, watch demos from years ago, not recent ones. Sometimes TF2 requires patience, something that todays players lack. Which I don't really get, since it's a part of the game. Don't know why people play the game if they dont have patience