One positive incremental change in the MMO world is the introduction of different character modes. That is, you can hit a button and switch the focus of your character. You can fulfill multiple roles, but not all at once, with a way to switch between them. Examples include Champions Online and DC Universe (no classes, just modes), dual talent specs and Druids in World of Warcraft, and the Minstrel and Rune-keeper in The Lord of the Rings Online™. If you have the skill points and cash, you can also switch ships in EVE Online easily enough, which would be like hopping classes in another game.
These vary in their ease or extent of switching between modes. The two main LotRO healing classes need about 10 seconds to switch modes fully mid-combat. My WoW Paladin lost all her mana when switching. Other games might require you to go back to town to switch, which is still nice although certainly not the one-click, mid-adventure thing I am talking about. The effectiveness of doing so depends on how flexible other aspects of your character are. In LotRO, you must visit town to change your traits, and I know how I hate it when our healer is traited for damage. In WoW (late game), you would want to be carrying a second set of gear if you switch from Retribution to Holy.
Another way to implement modes is to switch focus within a role. A Lord of the Rings Online™ Hunter has solo and group DPS modes, the former with higher threat and mana costs, the latter decreasing them but losing bonus damage. (Solo mode: good for pulling targets off the healer, not worth much else post-Siege of Mirkwood™.) Switching your Warcraft Mage from ice to fire is probably a less dramatic change.
While I love my alts, I am in favor of anything that will let you stick with one character. Let me stack all my options on one guy and switch which option I use, rather than switching between Zubon, Zuba, Zoobown, and Zupwn. While that will make hotkey management interesting, it saves me from having separate friends lists, guild rankings, vaults, key bindings… (You could also implement saved (and importable) or account-wide friends list, guild affiliation, shared vaults, key bindings…)
[Update: I see that Tobold just hit this theme from the POV of a DPS class in the post-LFG WoW world. Yeah, dual-spec does not seem like a huge boon for them. Having played ranged DPS in quite a few games, while I cannot address how WoW is this week, we are generally doing fine and soloing brilliantly, even if we are over-competing for group slots. I feel more for my healers, like my poor CoH Controller who fought bosses by putting his damaging hold (“stun” for WoW folk) on auto-repeat while I went AFK and waited for the pitiful DPS.]