No, it's not just you. The pre-Cataclysm patch 4.0.1 has indeed already begun turning the world upside down. Some caster DPS is far, far higher than it was a week ago, while melee DPS (especially DPSers who made heavy use of armor penetration) have seen significant shortfalls. Tanks are reporting threat issues across the board, as Wrath heroics that they tanked with ludicrous ease a week ago are suddenly providing them with much more difficulty, especially on groups of mobs. It's the pre-expansion shift, and it's wreaking havoc with players who've grown accustomed to how their classes or specs have played since November 2008.

You're not alone. To try and put things into a nutshell:
 

• With elements of play like the new spirit-to-hit conversion and new talents and abilities, some casters are reporting greatly inflated DPS.
• Meanwhile, with the loss of stats like armor penetration and tinkering with buffs like Windfury, melee DPS of all stripes are reporting DPS losses. Some classes are reporting as much as a 50 to 60 percent drop. Some of this is probably due tosuboptimal gem or gearing options. ArP's switching to crit like it did basically loaded up a lot of people with far more crit than they needed, as one example.
• Healing is in a weird place. Resto druids miss their forms, and across the board, heals are reported to be slightly less efficient -- but since no one's leveling past 80 yet, rating decay isn't crippling anyone.
• Tanks are reporting significant issues in trying to tank the current content with the new systems. AoE threat is greatly reduced for just about everyone, and learning new mechanics is causing some difficulty. For instance, if you spam Heroic Strike as a warrior tank now, you're doing it wrong. Armor has decreased in many cases, the loss of defense has cost tanks who relied on that stat some pure avoidance, and in general, the greatly inflated DPS of some casters has made keeping threat off of them extremely brutal.

Things are going to be up in the air for the next couple of months, more than likely. If your class got some of the ludicrous buffs, congrats on god mode until Cataclysm. If you instead suffered for the changes, my condolences; I do know how you feel. I really, really do. Whether we'll see adjustments before Cataclysm itself or not is unknown, but unlikely, as the focus right now is most likely on making sure the expansion ships in a more balanced way.