Lack of atmosphere and life. This a totally personal thing, but I find that as the GT series has gone forward in so many ways, it has turned into quite a rigid, sterile game.
GT5 could save the day with this. I can imagine car damage + good multiplayer features like private lobbies, track-day/free practice type events, owners clubs, guilds, auction house, paint shop etc.. making the game suddenly way more fun to play. I'm skeptical though, as this area doesn't seem to be PD's strong point.
The tame, tame racing. GT has so much appeal as a driving game alone, but I rarely feel like I'm racing. Again, good online-mode will solve this.
I've hated the menus since gt3. I hate the overproduction, I hate the aggressive synth jazz, I hate the slowness and being asked if I am sure. I used to love gt1 and gt2 menus - bring them back!
Attempting to include rally. I respect the ambitiousness, but I'm very very doubtful that it'll ever even come close to the best rally dedicated games out there. They should just team up with Colin McRae Rally and let them handle that part or something
The sounds of the cars. But I think this has just been a memory issue up until now, so can't hold it against them.
The maker is so disconnected from it's fans, and that none of these criticisms or requests will ever be read. This, however, goes hand-in-hand with another fact: The games are made by a man who is 100% passionate, and making it in his vision, and his alone. Despite the various things I hate about current GT's, I am VERY hopeful, because I think Kaz' vision is just as ambitious as any of ours, and lord knows he loves cars, and is probably the hardest working 'puter games maker the world has ever seen. Most of what I don't like, I know he's already working on it while I sleep, lol.
p.s. I totally couldn't agree more about the billiard board smooth tracks. The few lumpier tracks have always seemed the most fun and involving to me, but they are still missing all the little cracks and undulations that could potentially make the whole game come alive at your fingertips. But I couldn't say I hate GT for it's tracks - they have made some great ones.