A few quick observations, by no means complete and exhaustive:
FM2 looks better than I remember. OK, lighting is very flat but graphics aren't as aliased as I thought they'd be.
The improvements to overall lighting from FM2-FM3-FM4 are remarkable.
Silverstone's stormy sky in FM2 looks great and needs to make a return.
FM3's plain blue cloudless sky at the DLC Nurb F1 track looks terrible, like they forgot to add the skybox.
New York in FM3 is a lot more fun than I remember, and looks beautiful.
Sound in FM3 is better than FM4 for the simple reason you can boost the engine sounds from your opponents with a separate slider, so there's a real aural sense of what's around you, of being in the middle of a pack of snorting supercars.
Positano and Amalfi are sorely missed in sub-A class cars.
FM3 seems odd without active aero and flaming backfires.
Maximum 8-car grids in FM2 and FM3 seem really empty now.
AI seemed better in FM2 and FM3 than it does in FM4. More human, somehow.
FM3's skybox and rock formations at Camino Viejo are better looking than the updated ones in FM4.
Playing Camino's Extreme circuit in reverse again is great (why was this removed from FM4?)
The FM3 ultrawhite menus hurt my eyes.
FM2, FM3, FM4 (and probably FM5) all say 'saving data' when they really mean 'loading data'.
UI and overall presentation has improved tremendously across all three versions. FM2's UI is particularly fugly (but the music is great).
All cars I tried seem a lot more grippy in FM2 than FM3 or FM4.
I'd forgotten how limited the track (and ribbon) selection was in FM2.
I'd forgotten how awful (and how numerous!) the snake tracks were in FM2.
The overall impression I got was one of overall improvement from FM2 through FM4, but not necessarily in all departments. FM5 could benefit by drawing on the whole legacy of the FM series, and not just use FM4 as the starting point.