1. They aren't making the books all interlinked such that they presume you have everything ever published. I really hated in the TSR 2e era, buying a new book only to find it had plenty of places where it referenced books that had been out of print for years, and omitted key details assuming it would be just reprinting what you already had.
2. They aren't splitting up their customer base with their many products. In the 2e era, they had Forgotten Realms, Planescape, Spelljammer, Dragonlance, Dark Sun, Ravenloft, Birthright, Mystara and Greyhawk as settings recieving support. Part of the problem is that people would get one or two settings they'd play, and ignore the rest, and it split and factioned up the player base, and the product lines. If a really good book on being a noble PC and running fiefs as a player was made, it would almost certainly be for Birthright, which means players of other settings would pretty much ignore it. A psionics book would likely be for Dark Sun, and might well not be balanced for use outside that setting.
Now, while WotC does produce many books, and they have to to stay profitable and keep their masters at Hasbro happy, they do have a distinctly different take on things. They don't assume every player has every book previously made, just the core (or sometimes they put in material tying to big-selling books like Psionics or Vile/Exalted material, but at least with notes on how to incorporate this in a game if you don't have those books already).
They also don't split the product lines in many ways. Greyhawk exists as a generic default meant to be adapted and exported to other settings. Forgotten Realms is a classic flagship setting. Eberron is the only new one. They made a one-book release of Dragonlance (with future suppliments not published by WotC), Ghostwalk as a one-shot, and a remake of the original Ravenloft module (but not the actual setting). They also are much more likely to make a book that's meant to be generic and usable in any campaign instead of a setting-specific book.
Also, as was noted, WotC makes one very big, fundamental change that TSR never did: They listen to the market. They perform market research, polls, they listen to what the fans write on the message boards, they know that TSR died in large part because they assumed fans would buy whatever they made just because they were TSR.
