1. The drivers are slaves to the racing line, to the point of excessively slowing down to stay in it, rather than making a common sense decision to abandon the racing line in the pursuit of maintaining/increasing speed/position.
2. The driving line they are stuck in isn't nearly aggressive enough.
3. The driver pacing system is terrible. A "hot" driver shouldn't pace down to a crawl for laps 2 through 200 just because he has the lead. The only thing worse is watching him cruise around cold for 190 laps when NOT leading, just because there isn't another driver in his immediate vicinity. If you're not winning, you shouldn't need encouragement to speed up. The permanent 1 to 5 pacing scale used in GT4 was 100x better than this system.
4. The XP and leveling system used to unlock races is broken. Having a license requirement for event entry makes more sense than having to B-Spec 240 hours of a single endurance race just to unlock the last 24 hour race. Better yet, that last race is worth the most XP, but there is no level after 40 for those points to apply to. After reaching 35, you'll do 264 hours of B- Spec, and have 2 cars to show for it.
5. That last gripe wouldn't be too bad if not for the inability to speed up the time clock. These races require too much time to complete, and it makes zero sense to have them all play 1:1 against real time, especially when you don't even need to watch the race.
6. The reason you don't even need to watch the races is because B-Spec teaches you that absolutely obliterating the field in a vastly superior car is the best possible approach. The game does nothing to increase rewards for close/competitive racing. Winning by 40 laps teaches your driver just as much as winning by .4 seconds, so why drag out the race and unnecessarily risk/delay your rewards?
7. The drivers you train in B-Spec can't join you in your own A-Spec Endurance career. Why should I drive 24 hours at LeMans all alone when I have perfectly capable drivers I've been training to do the same thing? My B-Spec drivers can split a 24-hour race among three drivers, so why can't I? I would love to drive a 4-hour shift and hand the car over to my guys for the next 8 hours, which I could watch, or just skip up to my next shift behind the wheel. Using your own trained B-Spec drivers instead of some random level 0 scrubs (that would waste all your hard work by losing your lead) would make the ultimate incentive to play in B-Spec mode.