4e comes out, and yeah, it's fairly different than 3e, but it does have some cool ideas, there's been a ton of streamlining, it seems much easier to get into and get working with than 3e was.
But a series of flawed decisions, outright mistakes, and actual crimes crippled the game.
Let's address the most tragic: D&D 4e was intended to have, very shortly after launch, an impressive set of online tools: a virtual tabletop with custom design features, a character creator, monster designer, basically everything you would need to play D&D online.
The Senior Manager of Digital Technology Projects, discovered his wife had had an affair, bought a gun, and killed both her and himself.
This crippled the launch of the digital components, with the Virtual Tabletop never being realized, and the other programs being delayed.
One of the flawed decisions that plagued the game: a primary intent of the game was to be easier to get into for new players. Unfortunately, someone decided this should also mean, generally speaking, "easier". As such, the damage dice of monsters, and their health pools were balanced to provide 2 things: first, little risk of actual character death. And second: sufficient time for the players to use several of their "cooler" abilities.
This lead to fights that weren't exciting, because neither side was making appreciable progress. They eventually address this...2 years into the publication history.
Now, this could have been used, with proper marketing, as an upside. The release of D&D 3.5 didn't cripple 3e sales. They could have marketed it as a unlocking of the game's "hard mode" or so forth, releasing "optional upgrades" to the first few monster manuals "for the truly hardcore".
Instead, they just released the monster manual 3, using the new math, and tried to ignore the fact that they screwed up the math the first time.