I'm posting as a huge GT5 fan to talk about why I love it; as this is what the OP was asking for. Maybe this post will help build a picture of the average GT fan. First, to talk about my feelings on his key bullet points (as a way to contrast his opinion with a fans opinion):
The major part of the game is racing but it is not a racing simulator and other titles are better at racing simulation.
Gran Turismo is, and always has been a CAR + RACING simulator. So they have to strike a balance between satisfying the auto enthusiasts lust to drive every great car in the world, and their need to push them as far as possible on the track. Something like iRacing is a RACING simulator, but has a tiny percentage of cars. The first thing I did when I picked up GT5 is work toward perfecting my Ferrari 458; which I have a lot of personal interest in. It takes immense time and resources to secure licenses to the number and variety (and quality) of cars in GT5. I am thrilled to be able to drive my tuned 458 on some of the greatest tracks in the world.
It has very good driving physics but the cars are not set up to factory specs and require modification to achieve factory specs.
I think most street cars are almost perfectly set up to their factory specs. Some racing cars may be "detuned" like you mentioned...but this is kind of part of the GT "game". It can be frustrating at times, but tune ups are pretty easy. I guess my point is that this issue has never really been a problem for me.
It has a gaming > simulation aspect to it but the gameplay is often tedious instead of fun.
GT is a simulator period. If gaming is your priority, you will hate it. It is a simulator built within the constraints of a CONSOLE game though. Thus it is going to be a bit less "hardcore" than a PC only simulator. For example, it doesn't require a wheel. That took a pretty substantial development to make work reasonably well. I don't think any PC simulator on the market can be used at all without a wheel (you would fail instantly).
It also has to work within the constraints of an insane amount of legal documents. Imagine the team of lawyers PD must have to employ to secure licenses to every car they have in this game. I can imagine it taking 5 years of lawyer time JUST TO GET THE LICENSES. Every one of these manufacturers will be pushing PD to make their cars look as good as possible or no deal. If you wonder about damage modeling; well, there you go.
Aspects such as modifications seem to be aimed towards the casual gamer rather than the hard core driving simulation nut but the game is not structured in a way that is appealing to the casual gamer.
I think GT titles strive to do two things:
1) Pull in new people to the game by having a few easy/fun capabilities. Arcade mode, etc. People will either put it on the shelf, or will become addicted.
2) Really cater to the hard core GT gamer who will put 1000+ hours into every GT game they buy.
Personally, I think GT5 pulled this off better than any GT before it.
GT5 sacrifices simulation for hardware equality.
GT5 is a console game. It's NOT a PC game. All hardware is identical and equal. [EDIT: I just noticed 7HO's previous comment; he was talking about controller hardware equality. I guess I use my DS3 much more than my DFGT, so haven't really found any issues with the limitations. GT5 is definitely making me use my DFGT a lot more though, so that's probably a good thing.]
Why I love GT5: The best way I can describe it is auto porn. Sorry, but that's just it. If you are a hardcore auto fan; the guy with massive posters over every exotic car on your wall growing up...then GT5 is nirvana.
When I unwrap a new GT title, I'm looking forward to slowly working my way up. It's kind of like real life, except I'll make it further. In my real life I earned a Civic, then a Miata, and pretty much got stuck there (not that I don't love my Miata). One of my closest friends made it further. He made it all the way to a Ferrari 360, then Challenge Stradale, and takes delivery of his 458 in March. He does track days every couple weeks at Laguna Seca.
I know, in my life, I won't be able to do that. In my virtual life in GT5, I am collecting and driving cars I'll never get to own in the real world. And when I go to Laguna Seca with my friend on the first track day with his 458, I'll be able to give him some advice. The car on the track actually is not that far off from reality. You can practice doing laps in GT and it will help you choose your lines on the real track. At the very least I'll be able to talk intelligently with him about the problems with a particular turn.
I remember a period in my life when I had to sell my Miata in order to make my first house downpayment. I was extremely sad. I bought GT4 and used it as a crutch until I could get another Miata a few years later. It actually worked. I could take out my virtual cars and tracks instead of taking my Miata out on the twisties of Highway 1, CA.
When you play GT5, you just know the people who created this game are just like me. They are complete car nuts. Look at the interior shots posted elsewhere on this forum. That detail is there, but not even accessible at first!
I find the game play very good in GT. I've been so disappointed with so many of the AAA titles I've bought for the PS3 in the last few years. They cater to a very specific gamer, but are really limited beyond that. I could go on and on about why Medal of Honor or Black Ops was a disappointment for me and I don't play them much anymore. GT5 is not a disappointment, and I've put way more hours into it.
It is insane how many hours of challenges, license tests, and races are available in this game. Beyond that there are huge online capabilities that just keep growing. Just this week an update gave us "seasonal events", which will keep the game fresh every month. The community features are done REALLY well; better than any other PS3 game. The idea of a lounge you can just chat in, a log of achievements, a message board...these are things I've been begging game developers to include for years. It's so much easier to compare my stats to my friends than in other AAA titles (say COD). PD figured it out. I sure hope the other developers are watching and will copy.
I've bought most console racing games over the years (especially during the dry periods between GT releases). None have the same emotional appeal at the GT series. I own a 360 that only ever gets turned on for Halo and FM, but FM just has nowhere near the depth of GT. I've dabbled with PC racing; but while the simulation and graphics are great....no PC racing game has the financial backing to get the car licenses. This makes it a non-starter for me.
I hope this longwinded post helps shine some light on the audience PD was creating GT5 for. Sorry if you ended up with a game you don't like, but they made it for me. Now back to trying to beat that blasted X1 challenge :-D