I'm a big fan of the idea of feats that scale with Fighter levels... I really think they're great. However, my concern with the multiclass feats is that it makes things too dippable in the long run. Think about it... if Fighter feats scale with level, then taking just 1 Fighter level in your build combined with a multiclass feat suddenly makes your feats all scale (and you might as well use the Fighter level for that). That's a ton of power from a single level of Fighter, which I think is something to avoid.
Also, we run a huge danger of pulling the Halfling Outrider trick, wherein a single class advances tons of other classes (the Halfling Outrider advances Ranger, Druid, AND Paladin mount progressions, so if you have all three, you get a triple bonus).
I mean, imagine a character who has a Rogue/Scout multiclass feat, a Ranger/Scout multiclass feat, and a Fighter/Scout multiclass feat. Now his scout levels give him favored enemies, sneak attack, and feat advancement... that could get ugly fast and lead to a lot of builds like Fighter 2/Rogue 2/Ranger 2/Scout 14 while Scout 20 is extinct.
So, yeah, that's the danger. It could be dealt with a bit by making sure you can't qualify too quickly for multiclass feats, or by making sure that later abilities that classes get are much stronger, but we need to be careful.
JaronK