It is streamlined into each class being focused on certain roles, rather than the more free-form 3.5 system (where you could just take feats or take a level or two of a class that has the skills you need, followed by a PrC which further developed those skills). Instead, a Fighter is pretty much a Fighter, a Warlord is a Warlord, etc...
The Powers system is interesting, but at a glance it appears that the difference between a Mage and a Fighter is in thematics and range; Fighter "Powers" look more like spells geared towards a front-line magi, than abilities a Fighter would pick up. It does give the system a greater balance, but then they had a supplement in 3.5 which went a long way to balancing things out with the martial classes vs casters.
This is, of course, without playtesting it. So there could be some differences that a glance wouldn't tell. At a glance, however, I see nowhere that it is significantly different playing a Mage from a Fighter, and that is somewhat insane.
And then there are no Druids. At least in the core 3. Or Gnomes. How am I supposed to load catapults with no Gnomes to put in the baskets?
I think it's telling that we have at least one (poorly balanced from what I've read) 3.5e rulebook released that at least partially is because people don't like 4E. Then again, I remember the firestorm around 3E when it came out, and the 2E/AD&D players despised it, among the reasons being that it was "too much like a video game".
Then an almost greater firestorm over 3.5E, which was seen by some as a means to bilk people out of money (never mind most of the core changes are available in the SRD for free).
I'm sure when 5E comes out in some years, there'll be a firestorm over it as well. And 6E and 7E etc.
My thought is, the 3.5 books are still readily available on Amazon; go buy them and play it instead. I've been planning on picking up some of the ones I never got around to before. Particularly, the rest of the Compendiums (I love having the Spell Compendium handy. So much more convenient). If the demand is great enough for 3.x style books, then people will make and sell 3.x style books. Even if Wizards won't. Pathfinder is proof of this.