I'd never level a solo toon any more, and probably not duos. Heck, when my tank for my 5 man group got behind due to boosting, I used him to boost 3 warlocks and another paladin, to boost my tank... I know, confusing. But i've become such an altaholic, I'm already planning boosting at least 4-5 more configurations of alts (4x mages, 5x hunters, 5x shaman, 3x paladin, 3x rogue, 3x druid, etc etc) Right now my main goal is to boost my tertiary paladin to use him to boost my secondary paladin. After I finally got my first 70, I was like "I wonder what x class is like" but I just couldn't bring myself to level anything past 12. The thought of doing it all over again kept me from actually doing it. Playing 5 adds a challenge, some excitement, and such utility, that it makes the leveling part actually fun, even multiple times (I must have ran deadmines like 4 times with my group, and like 40 times with my 70 as a booster) and only after the 30th odd run did I get bored, then I moved to SFK Solo is definately the new gimp.

Solo play at 70? Maybe when I want to "think small" I did a bit of kara with a random pug, (just did trash, as 3 people never showed up, and people were leaving faster than replacements were arriving, oh how I wished I had 4 more 70s to fill in the blanks...) and I was bored to tears. It was a huge load off though, and I was able to surf the web much easier than when doing 5. When I don't want to do 5, I will probably 2-3-4 box different combos to level them up, etc.