It's 6:30 in the AM and I don't have time to read every post as I should be sleeping (insomnia, I haven't been to sleep yet...), but here is a short list of some major players that are missing:
Hyundai Genesis Coupe; E34 BMW M5; E30 and E36 M3s; Porsche in its entirety (I know, I know...licensing issue); Ferrari 288GTO, 250GT Lusso, Testarossa, F50, 333SP, and I'd have loved to have the 355 and 355 Challenge too (and I'm not a huge Ferrari fan either); Bugatti EB110; Koenigsegg in its entirety; Ultima GTR720 (fastest road-legal car on street tires around the TG test track [1:12.8], and on slicks it was faster than the FXX [1:09.9]. Some will deny it or try to discredit Ultima Sports Ltd but it's true); Lancia 037 rally car; MG Metro 6R4; Audi Quattro rally car, maybe even in Pikes Peak trim (the Quattro listed is a lame concept car from '03 IIRC); Mosler MT900; Noble in its entirety; any of the old McLaren CAN-AM racers, notable classic F1 cars like the Lotus John Player Special, the Brabham BT46B "fan car", Senna's McLaren MP4/4...the list goes on; old Aussie Ford Falcon XB; BMW M1; any Alfa Romeo with the word "Abarth" in the name (couldn't care less about the Fiat 500 Abarth personally, but the Fiat 131 Abarth would have been amazing); an Aussie V8 Supercar Holden Commodore to go with the Falcon XR8; an early 70's Mercury Capri; Alfa 155 touring car (might be on there, there IS a 155 but it's unclear if it's the race car); Aston Martin AMR1 (Le Mans racer with a life cut short); and lastly, I know I mentioned Porsche before but the 956 that set the STILL STANDING all-time N'ring record back in 1983 (6m 11s) would be ace...but I'd settle for a Dauer 962 (snuck in the same way they got RUF in there).
I know GT5 can't be all things to all people, but all of the above cars are significant whether it be because of its innovative design, its racing pedigree, or its popularity. The concept cars I can understand including because the point of GT is driving fantasy as much as it is racing. BUT, there are a lot of "econo-boxes" that were included in GT5 that would be well replaced by ANY car I've listed. Do people even really race in the Daihatsu Move? Cos I know they don't
fantasize about it

And let's not get started on the number of Miatas/MX5s/Eunos Roadsters, Evos, and Skylines there are...Granted they are legacy cars from previous games, and if the model exists you might as well include it.
The fact that I thought up all the cars on my list from memory says that PD has some gaps to fill, because by no means do I consider myself an expert. I love the series, I've completed I through IV all to 100%, and I will PLAY THE HELL out of GT5. I just hope there will regularly be DLC cars, and more love shown to European cars and classic race cars. Maybe then I could forgive the Sony bureaucracy for delaying it time and time again.