To me, the biggest let-down aside from lack of PvP is that there are no NPC trainers.
It would be sick if they had 'celebrity' trainers (like Brock, Misty, etc.) show up periodically and set up some kind of temporary gym where you could battle them for rewards. It would be even cooler if they were like, 4x normal Pokemon strength and had exotic movesets.
The lack of challenge in gym battling is a let-down and makes things just a straight-up grind with minimal strategy. (Which is why I find leveling up friendly gyms much more satisfying than taking down others teams' gyms. It's too easy)
The only 'hard' parts about gym training are trying to control errors/lag/GPS drift, making sure you have enough items on hand, picking the same g*ddarn team over and over 1 at a time, and praying to the heavens you don't get sniped - none of these things are fun...just annoying.
Lots of great ideas obviously floated around for PvP (which the game desperately needs), but they have to be careful b/c you know some perv will set up a gym to try to lure women/kids/whatever and ruin it for everyone
