These days, I'm effectively juggling three characters:
Greenwiz - When a useful raidID is available (generally Archavon, though I may add Sartharion 10/25 to the list after a quick and painless 10-man PUG this week), Wintergrasp if it's going to happen relatively shortly after I sign in, and perhaps the daily heroic 5-man if the Alliance holds Wintergrasp.
Lyriana (EQ2) - Tradeskill work when time is limited, questing when I have a longer chunk of time. These two activities have separate rested exp allowances, and I'll generally try to avoid doing them when I don't have rested exp.
Cheerydeth (WoW alt) - Very occasionally, when I feel like stabbing stuff and absolutely only when she has rested exp.
Green's issue is somewhat by design - the developers feel that the quality of the rewards I can snag from these instances requires that they be limited to once per week. I'm not sure if they necessarily meant that I should not play the character once I'm out of raid ID's, but at least there's a direct connection between what the devs wanted to do (limit the rate of loot acquisition) and the result.
Rested Exp, on the other hand, was theoretically implemented to help players who spend less time online keep pace with their friends who spend more time online, rather than to deter players like myself from playing past the end of the exp bar. I don't know that it actually serves that purpose in the current era of rapid level gain - when you're gaining a level every 2 hours, it doesn't take much time to outpace your less active friends (though EQ2 does allow you to turn off exp gain in this situation).
Neither of these incentives to switch are actually bad in my case, since I'm happy to play any of the above characters. (WoW raiders who have beaten all the current content may be less amused by the lack of anything to do beyond 2-3 nights of farming stuff they may no longer need.) Still, one has to wonder whether there are other ways to work around both of these issues - pace of loot acquisition and being able to continue playing with your friends - that don't push players who are paying attention to keep up multiple characters and/or games.