Full list of features so far, in quick bullet point form (thanks to goldrusher on gaf):
cars
• 1000 cars (800 Standard, 200 Premium)
- new brands include Ferrari, Maserati, Bugatti, Lamborghini, McLaren, Tesla, Red Bull Racing
- new series include NASCAR, WRC, Super GT, karting
tracks
• more than 40 tracks (more than 100 layouts)
- new real circuits include Top Gear Test Track, Daytona, Indianapolis, Spa-Francorchamps, Nürburgring, Monza
- new fictional circuits in London, Rome, Madrid, Tuscany, the Alps, the High Fens
offline:
• single race, split screen, time trial
• license tests, championships
• A-spec & B-spec
• car tuning
• 8 MP photo mode
• GTTV (compatible with PSP)
online:
• up to 16 players in a race
• private rooms, lobbies with up to 32 players
• spectator mode
• text & voice chat
• photo- en replay album
• YouTube replay upload
• time trial rankings
other features:
• cockpit view for Premium cars
• damage & roll overs (all cars)
• day-night cycle
• dynamic weather
• track creator / editor
• skidmarks & reverse lights
tech:
• up to 1080p 60fps
• up to Dolby 7.1
• custom soundtracks
• PlayStation Eye headtracking
• 3DTV support (gameplay & 3D photo mode)
also:
• all cars bought in GTPSP can be transferred to GT5 for use in Arcade Mode
• Adrian Newey was involved in a way
• Sebastian Vettel did voice work
