Granted, lots of speculation here, but I'm willing to speculate, rather than just shrug my shoulders:

1. He hinders the GM. When Hicks brought Buck in, I think that part of the reason he was excited about him was that he had the sense that Buck recognizes young talent. I don't think that he is terrible at it or anything, but he is NOT an elite talent evaluator, and his evaluations get in the way of the GM. Cases in point, pushing for Nix, Wilson to be called up obviously too soon, being unwilling to play guys like Castro, Botts and Laird. To make matters worse, the owner enables him by giving him too much power and say.
2. His managerial style is very tiresome (many words could be used here). By "managerial style" I don't mean on-field managing, I mean his handling of people. However you feel about Buck, I think that everyone probably agrees that there is more than one way to manage people. Difficult managers can be very successful, just as successful in measurable short term results as those with other styles. But their message often expires. And I just don't see how a fan of this team could miss the correlation between when the pressure is on or off, and how it plays. It takes a special player to handle the added pressure someone like Buck contributes, and it is just too hard in this day and age to try to build a team with the best players - and the ones who can play for him. There just aren't enough on the planet.
3. He does not communicate with his players. He uses the media, hints, any passive-aggressive means to get messages across to his players, instead of being up front and honest. This is an area where if you want to bury your head in the sand, like some, you can. But using any clues that are available, and some really good ones are, it is easy to conclude that whether it is really the case or not, his players find him unwilling to communicate. If there is one characteristic that I think that players in this day and age ask for from a coach, it's communication/honesty. I was astounded when Parcells got to town at all of the players who touted him as a manager, even guys he had fought with. I mean Antonio Bryant is even doing it now. And what do they all say they appreciate about him? They know exactly where they stand.
4. From third party claims, Hank Blalock and Mark Teixeira despise playing for him. Now first of all, this could easily go into one of the other headings, but these are players who, before regressing under Buck, were among the best young players in baseball and were players that we had all hoped this franchise would build around. Teixeira still is, but he's gone if Buck isn't, and he hasn't played like one of the best in baseball this year.
5. He prepares VERY well, but that preparation is negated to a large degree by his biases. What good is getting to the park at 10 a.m. every day to know your stuff if you still choose Barajas as your catcher most days and feel the need to insert Jerry Hairston into corner positions regularly?
6. His management of the bullpen has slipped in the past two years. One thing that I found very refreshing about Buck early on was his handling of the pen. It always seemed like he had a better feel than some of our previous managers. But for part of the last two years, and most of this one, I find myself wondering why he leaves guys in, why he allows bad matchups, etc., and quite often those curious decisions have lost games. It is important to note that the reason for this isn't that the bullpen has gotten much worse. His handling of it has just undoubtedly slipped, and I'm honestly not quite sure why.

These two factors remove much of the in-game reputation that he has garnered from Jamey, etc. He is an average in-game manager at this point - not a bad one, but certainly not an elite one.

7. The team's best players - and not coincidentally the ones who were (I say "were" because some are gone now) here the longest have all had poor seasons - in most cases their worst. Call it coincidence if you want, but seemingly every source agrees that players tire of playing for Buck. And Blalock, Teixeira, Young, Cordero, Mench all faltered this year, in their fourth with him. I think that a certain kind of player does excel with Buck, and those players do tend to be gamers. But you can't win with a team of Dellucci's and Barajas's, unlike what some think. And Buck can't seem to manage anyone else.

To tie it together, he is following the same pattern for the third time now. He has had an owner/GM buy into him and give him lots of control immediately. He has demonstrated enough competence early on to confirm that latitude. But rather quickly, before he has been able to take his team to the postseason, his style alienated and disrupted his organization and clubhouse. In Texas, he was first unable to deal with established veterans like Rafael Palmeiro, Pudge Rodriguez and Alex Rogriguez and oversaw their exits. It was presumed that the new crop of stars would be more to his liking, but rumors now abound of very unhealthy relationships with Teixeira and Blalock, for two, and formerly promising players like Mench and Laird have stayed in his doghouse for a matter of years, presumably never to leave.

He cannot resist playing leverage games with the GM, and he will not be a positive force this winter as the team faces critical directional decisions, since there seems to be pressure to do something about Blalock and/or Teixeira, and there is a looming potential for him to relegate whoever he cares to from what the GM brings in, in the same manner as Laird and Castro.

And speaking of Castro, isn't it funny that the reason that Evan Grant bought that it was okay for the team to deal Castro was that it was in a "pennant race" and Philly was not. Yet who is one game out of the playoffs right now, and who is ten? Yet somehow they manage to retain him.