To be completely fair to both of you, tab targeting was never enough to maintain threat on a full pack in which the entire group was going balls to the wall with offensive aoe abilities, even "back in the day" when tabbing was standard practice.
Your aoe threat tools, perhaps with the occasional tab, were enough to keep a lead on your healer generating threat, but if your dps wasn't focusing on one target there was going to be issues, end of story. Heavy aoe was reserved for specific packs that warranted it (such as the arcane dudes in Nexus; 1-2 elites, lots of nonelite. The nonelite aren't going to kill your aoe folks if they hit them), usually because of some mechanic that demanded they all die together or that they simply needed to die without a lot of tank interaction (often because he can't effectively mitigate any of the damage from them for whatever reason).
So, I'm sort of in Mazra's camp on this; tanking is hell right now. The game doesn't warrant the use of heavy CC yet, and tabbing around doesn't produce reliable results when people can put out a large portion of their dps using buttons that deal extremely heavy damage to multiple targets simultaneously. You almost always feel like you're behind the curve. In a move that foreshadows the new expansion, aoe threat tools were heavily neutered to be a "maintenance" issue, and not a mode of attack. Extra tools are being provided at the cataclysm levels for general purposes not necessarily directly linked to aoe threat, but they serve that purpose in part nonetheless.
The dichotomy of the dungeon is expected to change, and people will be heavily encouraged to use their CC abilities. So in that regard, I agree with you, Xsarus; we're slowly going to go back to an age where tabbing between targets is a definitive part of the tanking experience. But that's in a week, and the period between the release of 4.0 and 4.0.3b or whatever has unquestionably fallen into a slight oversight with regards to the particular effect the new mechanical design would have on the current content experience.