#1. Too many dungeons. (Or, I declared a main three times this expansion which is also a terrible idea). Over WotLK, gathered most of my emblems through heroics. Each tier, I jumped back into five mans to farm those daily new emblems and dozens of old emblems I didn't need any more on my main, while my alts jumped into those heroics to farm the old emblems to get themselves caught up. For too much of the expansion, I was doing the same daily three different times. Never again.
At the start of the expansion, Mayhem and I did a lot of heroics. A lot. We put together groups to get specific drops. We had best-in-slot gear from heroics. I think the one mistake we made in all of this was that we spent way too much time "preparing to raid", without realizing that the best thing we could have done was just jumped into Naxx10 when we had badge and crafted.
69 Cyanigosas? That's too many dungeons!
That's not to say that doing heroics of some form won't be necessary or helpful; however, I will not be grinding like I did. I ended up each tier with unneeded badges on my main, because I pushed too hard at the beginning to get all that badge and crafted gear ASAP (and then I pass on raiding drops. I may have the highest PR in the guild, but nothing to spend it on). I will be more patient with gear in Cataclysm.
A part of this mistake is how much time I spent gearing up my alts to raid and then not raiding with them. My druid is all but retired now, after grinding her gear up to raidability she went into ICC10 and 25 a mere handful of times, totally disproportionate to the time I spent getting her ready for it. That is my fault, I won't make the mistake again.
That's not to say I won't be keeping up three characters in Cataclysm (I still ponder working on my shaman so I have all the healers, and I have a young mage I'm obsessed with). But I didn't play my mains in Wrath as well as I could have, and I'll spend more time playing and less time grinding to play. Since LFD has become a major part of the five-man experience, what heroics we do will be spent doing the instance instead of waiting around in town looking in trade for more dps. And when I'm done with a heroic, I'll be done. Find someone else to heal; 69 Violet Holds is too many.