From the upcoming PSM3 Gran Turismo article:
All of these paragraphs are direct excerpts. I left out most of the information we already know.
Cars
Of the roughly 1000 cars in GT5, about 800 are standard models. Mostly brought over from GTPSP, they're shiny, HD and still more detailed than any vehicle in Prologue. These vehicles only suffer scrapes and scratches, but it's crash consequences that matter. A trip to the gravel trap - or worse, the metal railings - in any car could prove race ending. Think broken gearboxes, brake failures and even engine stalls. All cars regardless of type have fully functioning front reverse lights, horns and get dirty during races.
Dynamic Weather/Time
Snow is similarly impressive, hitting the windscreen in thick clumps before melting into water and being wiped from view. Flakes start to clump in thick grass, tree branches and tracksides before slowly setting onto the track, one lap at a time. It's not just how stunning the weather effects look, though, but their unpredictability which most impresses. Weather even alters the temperature, air pressure and humidity on the track. Forecasts are available for each race, but in a cruelly realistic twist, aren't always accurate. During long races, clouds move across the sky, darkening the track and forcing a constant re-assessment of your race strategy. Failure to read the changing conditions could mean the difference between victory and crashing out.
Daylight alters during longer races, too. Night slowly sets in, reducing visibility and necessitating the use of hi-beam headlights over normal ones. In a 24 hour race, the sky's hue changes slowly fades to black and back without any animations. Stunning. In keeping with the hyper-realism, Polyphony have even included 'time-syncing' on real-life circuits. This allows you to match the time of day in-game to that of the real track - meaning if it's night time in Monza, you'll have to race in the dark. Yet another unnecessary detail that only GT offers..
Soundtrack
Just like the PSP version, you'll get the chance to play whatever music you have on your hard drive, so you're only limited by the size of your disk and the range of your MP3 collection.
Customization
You can tell when you're tailing a car with a racing exhaust rather that its standard factory parts, because it looks bigger and sounds louder than normal. You can tune electric cars. Tuning menus are more interactive and visual that before, too, with a much slicker user interface in the garage - though there's no livery editor this time.
Rally
Each race in the rally career is run on a randomly-generated point-to-point course with the aim, like in real rallying, being to beat your competitors times rather than race them head to head. Every rally is also affected by GT5's stunning weather engine, too. There's even a co-driver calling out each corners severity and suggested approach.
Nascar
NASCAR in GT only comprises a few 'official' tracks - including Daytona and Indianapolis - and roughly ten real drivers. The license will also play to the real sport's rules, such as yellow flags and pitting. The NASCARs are also the only vehicles which have bonnets break open and rear windows shatter.
Excerpts from Interview with Kaz
Dirt Track on the online mode, you can drive with many more cars that way. And the Rally mode is different again; each car is on a time trial, staggered by 10 seconds.
Wet-weather pit strategy can make or break your race. You will have to make the right decisions at the right time, and learn to be fast in both wet and dry conditions. This, with 16- player, private online race events, is going to be unlike anything we've seen before. I can't wait.
**Happy Dance!** 
