The gang on the Multiverse did a rundown of upcoming MMORPG's this week. They noted that there seems to be a general trend of backlash against class-based games in general and the "holy trinity" in particular these days, and they blame the rise of solo play. I'd suggest that their cause and effect may be reversed. In my view, the holy trinity mechanic complicates the process of looking for groups to point where developers are forced to offer more solo options as a concession to the difficulty of finding a group.
As the gang reminded me, the forthcoming Guild Wars 2 will supposedly eschew the traditional dedicated healer class, instead giving all character the tools to watch their own health bars. This might sound like a way to address the problem of the holy trinity, but I'm wondering that the devs may be giving players what they say they want instead of what they actually want.
Causes of "LF2M tank and heals"
In every game that I've played, the most common difficulty in assembling a group is finding players to fill the tank and healer slots of the "trinity". People who are down on solo play will jump to blame it for this problem - DPS characters often solo faster, they would argue, and therefore the system encourages players not to play tanks and healers. The truth is more nuanced than that.
I only lasted a bit over a month in FFXI back in 2006, which was about as solo-unfriendly as games have ever been. The tank and healer shortage was in full effect in that game, and I'd routinely see groups spend so long looking that the four bored DPS would try asking more DPS with tank or healer subjobs to try and fill the missing roles (which tends not to end well when the group also insists on trying to pull the toughest possible mobs for max exp).
Meanwhile, over in WoW, the fact that it's easier to level solo is nigh meaningless, because dual spec allows players to switch from the best solo spec to the best tanking/healing spec at the literal touch of a button. As Spinks points out, there are other issues involved in picking up WoW tanking at this stage in an expansion cycle (chiefly the learning curve), but I don't think you can argue that solo leveling alone accounts for the fact that tanks get groups nigh instantly, while DPS wait for 15-30 minutes.
The dirty little secret is that DPS IS EASIER. As a DPS, you need to know two things: what order to push your buttons in, and where to stand. The order in which you push the buttons may vary slightly based on the situation (perhaps you're saving cooldowns for a burn phase, or AOE'ing adds), but that's usually not that unpredictable. The where to stand part means being in range of the boss and not standing in the fire, and even the second part of that role is more than many DPS (myself sometimes included) can handle.
As a tank or a healer, you still need to be aware of the two things DPS need to know (what buttons to push, and where to stand) but you also need a far greater awareness of what the other members of the party are doing. I was once the last player capable of removing a curse from the main tank left standing in a 40 man raid, and that one minor responsibility - far less than a real healer would have to handle - was enough to make that fight the most stressful experience I have ever had in an MMO. Being a tank or healer is harder, carries more responsibility, and many players simply do not want this level of complexity to their hobby.
Distributing heals, responsibility
So back to GW2's little revision, in which everyone has to heal themselves. The practical effect of this change is that, instead of one player shouldering the responsibility for everyone's health bars, everyone has to add their own self-healing on to their other responsibilities. If I'm right, this means that GW2 DPS WILL BE HARDER than DPS in other games due to the additional task. The really good DPS, who always top the meters and move out of the fire and do whatever misc utility their classes have, will really shine under this system. Those of us who struggle to react quickly enough with someone else watching our health bars may not fare so well.
The point of asking for the removal of the trinity is to make it easier to assemble groups. It's simply not fun to have five people lined up outside a five player dungeon only to be told that they all have to sit on their rears because none of them is a healer. However, the new problem may be that this system further emphasizes the difference between a good player and an average one. The average player no longer does average DPS, they do 0 DPS because they failed to watch their health and they died. The irony is that this may leave players - especially the good ones - unwilling to do PUG's at all. If that's the case, a change that was intended to facilitate grouping may actually make it more difficult.