Okey, I took few minutes of thinking and made this list from my head.
Notice it is not list of anything new in GT series, it is only a list of features/assets we already had in previous GT games. Also notice I did not repeat the same features throughout the span of the series but instead opted to note them only with their initial appearance in particular game.
And also notice I did not included option to speed up the B-Spec races although it existed in GT4. I personally and subjectively find real-time B-Spec of GT5 the only good way to go since I actually find it a standalone real-time management mode with AI-behavior-development and not a way to quickly make money or easily finish endurances. So, when I mentioned B-Spec driver in GT4 description below, intention is to have him jump-in and drive his portion in the real-time.
Grand idea is to have this features from previous games included in current state of GT5.
So, it goes something like this:
Gran Turismo 1
- separate music tunes for all segments of the game (including dealerships)
- laptimes and records tracked throughout the game
- scrollable list with all laptimes after the race
- strict restrictions for races
- SS11 with underpass
- GUI shortcuts
- 1-400, 1-1000, max speed tests
- Memory Card Battle

(2P races should be called like that for nostalgia reason)
- single-screen replay for 2P races (abandoned in favor of split-screen replay from GT4 onwards)
Gran Turismo 2
- greatest Time-Trial options ever (sector monitoring, 3 types of ghosts, etc..)
- best Race Analysis tool in the series
- ability to rearrange cars through Garage
- Tahiti, Midfield, Red Rock Valley, Rome, Rome Night, Apricot Hill, Grundenwald, Seattle, rally tracks..
- monitoring and tracking highest speed per venue
- Event Creator tool
- restricted career separated through "GT World" and "Special Event" races
- separated Arcade mode with Unlocks
- skip-a-lap option in Replay
Gran Turismo 3
- real-time mileage-monitor on HUD
- Time Trial Arcade races with predetermined conditions
- Complex String, SS11 with overpass
- when Original Tracks are raced in Reverse-configuration the lightning (time of day) changes
- save-slots for tunes
GT Concept: Tokyo-Geneva
> no real details except for the PodRace for lulz

- oh yeah, animated leafs on Autumn Ring track
Gran Turismo 4: Prologue
- Coffee Break tests
- License Tests structured with skill-tests
- aerodynamic/engine stall effect in high speeds
Gran Turismo 4
- stock-version of cars is unlocked for usage in Arcade/LAN multiplayer when bought in Dealerships/obtained through Prizes
- complex career with World Map
- Dealership Races
- LAN option
- old-school versions of some tracks, Motegi, Seoul, Hong Kong, Paris, Cathedral Rocks, El Capitan, Costa di Amalfi, Citta di Aria..
- interchangeable position with B-Spec driver through pits for Endurance races
- 2X fast-forward in Replay Theatre
GT5: Prologue
- leaderboards
- official matchmaking with predetermined entry-requirements
- animated pre-race garage sequences for particular car/track
GTPSP
- simple yet great Event Generator
+ all License tests were non-assisted in all GT games prior to GT5 > in GT5 many of the License tests?Special Event races have SRF mandatory enabled and I personally find it insulting..
BTW I probably forgot something in lists above, but those are details I could remember on the fly, without booting old games separately. If I did, I presume lists would be even longer.