I see two related possible issues:I once had an idea of making all of the feats in a chain only require a single slot. Essentially, you bought the entire chain for the "cost" of one feat; the caveat was that you only got the benefits of the subsequent feats in the chain as you met their prerequisites. It essentially made the feat you selected level up alongside your character.
Of course, I never got a chance to put any of that into play, so I have no idea how well it'd work at the table.
1. In many cases, the only prerequisites for a feat is another feat (and possibly another before that). So you'd basically get the whole shebang at once.
2. It is also common that a single feat is the "root" to multiple different "branches" on the feat tree. For example, Combat Expertise is the gate to most of the Improved (Combat Maneuver) feats. You might not want someone to get all of those for just one feat.
I don't think either of these issues are dealbreakers, really. They just mean you'd have to rebuild the feat trees with that in mind.