Blood DK/Warlock is my personal favourite. You can tackle anything with that combo, though Blood DK/Hybrid Heal/DPS works even better, maybe. I can see Blood mass pull + Ele Sham AoE being quite potent for instance.
Ya I see what you are saying but obviously theres gotta be some combinations that are simply better than others. But ya I depends on what you're trying to do. I think tank/healer also is beneficial if you wanna run a dungeon so you don't have to wait forever.
When my husband and I quested together we had a Pally (Ret) and Hunter combo. My pet tanked and him and I did damage. He has since made a Lock that is his main, so next xpac we will be leveling with it instead of the Pally.
The warlock brings a tanking pet strong engouh to hold agro on packs, and is powerful as a DPS
The owl brings good simple dps, powerful controls and utilities, and strong support heal in case it's needed.
And yes I love multidotting while fire and storm rain on all those foolish enough to stand in our way !!!
I always did pretty well back in the day with a fury warrior (me) and a ret pally (guildy/friend). We tore through pretty much everything.
Actually, I tend to be pretty one dimensional and stick to a single class as my main. My friend, on the other hand, likes to alt hop and even do crazy things such as get a character to 90 then delete it to level one again. So, seeing as how we play together a lot, I've ended up running around/questing with pretty much every class he's played. From what I've found, we tend to do better when he's either on one of his plate DPS/tank classes or any class that can heal me.