If you've played MMOs for any length of time, you've probably grown very familiar with the trinity. You can't help but be acquainted with it in any sort of group content, where in more games than not there's that split between the tank, the healer, and the DPS. Certainly there are subdivisions and extra roles, but for the most part those extra roles are slight twists on one of the existing roles. (Controllers in City of Heroes are DPS with a debuffing/controlling aspect and slightly less damage, for instance). Brian "Psychochild" Green has an article on Gamasutra examining the issues with the holy trinity in game design: what purposes it serves, what its drawbacks are, and how useful it might be to get rid of it entirely.
Examining the roots of the trinity design in Dungeons & Dragons and common gameplay types it offered, Green goes on to take a look at how the structure has become codified, what other potential systems could be put in place, and if there's even an advantage to doing so. He discusses the issues of hybrid classes, group versus solo design, and proposes a potential alternative that remains rooted in the core elements of the design. Anyone with an interest in design should take a look at the full article, as it contains some interesting insights and analysis of one of the core underpinnings of our genre.