Wellll, Reasons and Persons is quite brilliant and fun. But first of all, for me personally, the ethical orientation - recycling Sidgwick's utilitarianism, relentless and fantastical emphasis on rationality - is boring and really not actually about human beings. (I'm going to say the same about Christine Korsgaard, for example: ethics for aliens.) And I think that On What Matters is absurdly overrated: his reputation and personal interestingness kind of stupefied people. First of all, this thing where he sent the manuscript to a couple of hundred philosophers, then partially constructed the book as answers to off-text objections, makes for an extremely labored, interminable, and at times bewildering presentation.
Here's how I would state the conclusion: teleological and deontological ethics, Sidgwick and Kant, are not as different as you might think and often yield the same results, etc. It tells you a lot of what you need to know that that makes it a bold, original, epoch-making text in ethics. It's really a kind of unreadable adjustment of the going positions, best at solving little conceptual objections that are very inside baseball. He was a wildly fascinating person, but the reception ('most important book in ethics in a century,' etc) is both wrong and a pretty good sign of where we're at.
Another sign of where we're at is that you just can't say stuff like this, though I would suggest that a lot of people who tried to read On What Matters ended up secretly having these sort of misgivings. Academic philosophy now is characterized by an amazingly stultifying little pecking order; philosophy is thoroughly 'professionalized,' and deference to one's superiors is a condition for getting in the door. Philosophy now is above all careful: I'm sure you don't want to put forward a fallacy, but you really, really don't want to put forward something challenging or even odd, because that's liable to deep-six your career. Basically positions are taken on the basis of safety and expressed in a way that reflects that. I do think it was different in the generation before mine, and if you think Richard Rorty or Arthur Danto would have had trouble saying things like this about someone like Parfit had they thought it, you don't remember. That was fun.
A straightforward economic analysis gets some of the reasons. The academic job market has been so tight for so long that most people's question is just 'how do I have a career?' It takes you 8 years, you've got a shitload of loans, your in-laws still don't understand how you can be doing this. You've got to pay off, so you clutch on by your fingernails and dedicate yourself to cultivating useful mentors, displaying competence, not making a mistake. My feeling is that philosophy is incompatible with reverence or with this sort of professionalism. And I don't think we're in a good era for interesting or original work, with exceptions of course.