It's not a matter of difficulty, it's a matter of focus. The central hallmark of what I thinking of here is how the class abilities have become principally about, and balanced around, combat.
EDIT: Simon Atavix and Andor also reflect perceptions I share.
I get what you are saying. My first impression of the game was not terribly different than yours. I'll be one of the first to say that the page after page of powers in the 4ePHB seem to be where a lot of the focus is.
What changed my perception was putting together some characters for my wife and daughter and playing the game some*. My understanding came to be that the Powers, which really only function in combat, were not the vehicle by which the exploration portion of the game came about. Exploration is largely handled via other subsystems in the game (mostly Skills and, to a lesser extent, Rituals).
The games I ran felt incredibly similar to my 3.x games when it came to the exploration part of the game. Sneaking, picking locks, searching for and disarming traps, forcing open stuck doors, figuring out riddles, all of that stuff that comes together in my mind as "exploration" worked nearly identically to 3.x.
It was really in combat where I could see most of the differences. And, to me, these differences were mostly to the good. It felt as though the powers were really just a more codified way of doing a lot of the stuff that was done in a standard 3.x combat (i.e. damaging, penalizing, incapacitating or moving an opponent, healing or buffing your allies as well as things like altering the battlefield in small ways).
Looking at your example of the Thief/Rogue, I still see them performing all the same original functions of that class in terms of the whole picking locks/disarming traps kind of dungeoneering. Just with the added benefit of being able to do more damage in combat than many other classes. I think the focus of the class is still enough on the idea of dungeon exploration for my tastes.
Anyway, I'm by no means trying to talk you (or anybody else) out of your position. Only saying that it took me a long while to grasp the fact that, with 4e, I could really run just about any game I wanted that earlier versions of the game could run. The tools are there for me, regardless of what parts of the classes take up a bigger page count.
*Don't interpret this as me saying that if you made characters and played the game that you should have come to the same conclusions as I did.