I think "keeping up with the Jones's" covers a majority (not all of course) of what the fans want. Now I have never played FM4 but I do lurk on ForzaPlanet. It seems if PD were to offer content similiar to that available in FM4, we would see a lot less wish list threads here.
Go for a quick lurk, do you notice a difference in atmosphere and dialogue compared to here in GTP? (Given the 16 months of GT5 versus the 6? of FM4).
Yet the same could be said for PD alone, they seem to have a few things that bind them, Sony's time line, being in line with Nissan (purely PD's making), watching what other race games do, trying to listen to the fans, and PD's own vision.
From what I've seen is that there is more people wanting PD to have diversity in cars and tracks. As well as better physics and a tire model along with a more creative freedom so racers can distinguish their cars from the hundreds of thousands that play. There are a few other things but none of these are to keep up with what NFS, T10, PG (were), Codemasters or any PC sim game is doing.
When you look at cockpits and weather you see it more of them trying to catch up with being behind the times. Same goes for the damage, but the switch from the unique license setup to leveling up instead is an obvious take away from certain other race games. That to me seems to imply that PD feel they need to mask to a small degree what others are doing.
As far as site content goes, there are so called "fanboys or diehards" that believe their beloved game could never do anything wrong. Or has always been on a linear path to greatness, sadly this isn't the case for either game. I do feel there are less wishlist in FM area, but that might be due to the game being out much less like you said. I see the wanting cars thing to be a double edged sword. FM has more popular cars and a better region ratio than GT, but with adding all the cars people want, the players hunger grows and since they add all these great cars a higher demand may occur. PD on the other hand really didn't add too much new, compared to what they ported over so it has the reverse effect but same outcome, a majority of what we played and raced in GT4, so now we hunger for more diversity and fresh cars to the series not just the game.