I like the way that in GT4, if you went to GTAuto you could tell how minging your oil was, even if the light hadn't come on in the race. I remember doing a Japanese "championship"-type event, and my oil light came on during the last race. I struggled to bring home 3rd; needless to say, I didn't neglect my oil after that! The same could apply to brakes and other components (suspension damage is supposedly modeled, or at least it was a target at one point).
I see clutch and brake wear as a similar thing. These only really need sorting after a race, but you'd best keep on top of it during a long series or championship (reminders would be cool, with a toggle, of course). Look after your clutch, and it should be fine during a race as long as you're not too stupid, and assuming the clutch control is improved over GT5

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Brake temps are something that you ought to be able to feel via the fluid temp, and hence pedal feel. That's not practically possible in a game: maybe adding brake squeal at the onset of fade could work instead, or (excess) vibration during braking, representing the irregular "glaze" from the hot pad material on the discs (I know it's not necessarily realistic, but the only other option is some sort of bar) - then you'd know to take it easy. Of course, the effects of fade on your braking distances ought to be obvious enough...
I've read that stock vehicles do have adequate braking performance, once the discs (design) and pads (material) are upgraded and (preferably) the fluid changed - the calipers themselves are usually fine. Some of my friends have testified to the same, but I've not experienced it for myself. The trouble is, these performance packages tend to be noisy!
I also remember, with GT4, thinking the chassis reconditioning and rollcage were gimmicks, but I was impressed by their effect in GT4. Some cars could be "ruined" with a rollcage; I "refreshed" the chassis on my R32 GTS-t Type M and completely rendered my "perfect" setup useless, so I had to spend a lot of time re-tweaking it, but I think it felt better after that!
It's things like this that make me think that if PD do decide to include features such as brake and clutch fade and wear, it'd be done with similar subtlety and depth.