I think the worst timesink is looting on five different toons. This adds up and is why many people don't loot when they are leveling up. For me, I have to switch mice and fumble around with my view on my second monitor to loot. I always have to get around my big-arse Tauren druid or some other toon in the way...move them etc.. Not to mention having to track who has what completed and to make sure everyone is done before I move on - typically this isn't an issue, but I have on occasion missed 1 item or a click and had to go back. These things annoy me the most. Drop rates, having to kill a lot of mobs or waiting for respawns (usually can kill other things in the meantime) doesn't really bother me. Of course, if I had to collect 50+ of an item I probably wouldn't do the quest.

The respawn and drop rates in WoTLK are pretty decent IMO [so far]. Of course, I only just got to Northrend last Sunday so it's pretty empty in Howling Fjord so this may not have been the case for many of you.

Mob availability can be an issue for sure. Obviously all those people solo questing will result in this. However, this is highly dependent on the current state of the game play. When the expansion was first out, this was probably insane. Now, that many people are 75+ it is less of an issue. The exception probably being dailies. Of course, the advantage we have is that we can take on multiple mobs at a time, while the solo player may not always have that option. Now, as you stated if they are all spread out, well..guess we are SOL on that.

I definitely could not agree more that grouping should not be penalized. However, I would assume that how they design the game [esp. expansions] is dependent on how people play the game. People don't group for leveling, so I don't see why they would design it to be as such. It's counter-intuitive for an MMO, but look at us....we are the prime example of solo play. Also, keep in mind I don't think it would be a good business decision to make leveling up efficient by any means. Why would they want to do that? It has to be balanced or the game will fail. Never disconnect the business from the gameplay. It will always be a business first.