I frequently quest with a friend as well. In fact, we often level a duo with complimentary skills that we only play with each other, ensuring that they're on the same quests at the same time. For us it isn't so much the respawn of items that's irksome, or the need to kill two sets of mobs, but the irregularity of whether we can enter quest instances together. Sometimes we can, sometimes we can't, and now in HD there have been a couple of times where we can enter together but we don't both get credit for quest completion, thus making one of us do the instance again and effectively negating the time-saving aspect of entering the instance together. Rawr!
"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world." - Thorin Oakenshield in [i]The Hobbit, or There and Back Again[/i] by J.R.R. Tolkien