Now, let me address your points.
* 24 Hours of Le Mans - Maybe due to licensing deals, PD can't use "Le Mans" in the game. There's licensing issues that take place. Would you use Pepsi in a game and not get a licensing deal? You'd have lawsuits against you if you did, so this stuff aren't gimmies. Just like... some European sports car companies.
* Silverstone and Brands Hatch - now, I'll agree with you. These are two awesome race tracks. Brands Hatch has the simple configuraion, then the much tougher long course. On the record, there has NEVER been a British race track in a GT game.
* Maybach Excelero - I don't know about this car, so no comment except for potential licensing issues.
* Official F1 racers - doesn't really matter much. It's okay to have fictional race cars because GT isn't about single racing series- not even the JGTC/Super GT. Then too, some just want to have a Ferrari F1 car and own everyone else. Then too, I don't care for licensed racers as well. GT has always been about the cars, not specific series and not specific drivers.
* Official tuning firms - to what extent? Bodykits? Special horsepower upgrades? Original engine parts? Clarify a little more, please.
* Pit stops with visible drivers in pits behind you - this may actually happen in the next GT with the PS3's power, so I wouldn't worry about this.
* pit communication - this MAY work, but many would complain that it can be a nusiance while racing. Co-driver commentary for rally racing would be a nice consideration, but that's about it.
* Create a car - Why? The original Sega GT did it pretty well with the Carrozzeria feature, but I don't really see this happening too much work and perhaps unnecessary if you ask me.
* Choose exhaust shape from tuner shop - No way. I would like to maybe see a rendered muffler package, so when you add a Racing muffler, you don't get sort of a look-a-like muffler for each application.
All in all, what's going to be rubbish about the next GT? It will be rubbish if GT5 ends up being a dissappointment to the GT series. Everything people complained about GT games of old (even some of mine) are stuff I read, take to heart, think about, and prepare counterstatements at will. Twenty-four hours is still 24 hours, even if the graphics don't show it. It's a shame compared to "Le Mans 24 Hours" (great game. Get it if you don't have it already), but the race is still a twice-around-the-clock classic. To me, very little will disappoint me unless PD just do horribly with the next GT.