Zubon, of the award-winning Kill Ten Rats cooperative blog, has a great design discussion up on on the site concerning the 'proud nails' of City of Heroes' elder game. The term 'proud nail' is one that Zubon and I both enjoy from the design discussions on the official Wizards of the Coast website. Wizards makes the tabletop RPG Dungeons and Dragons, and many of the observations they make there are easily transposable to Massively Multiplayer games.
Proud nails are 'design snags', problems that screw up the smooth movement of design mechanics. On the D&D site, examples include 'ten foot square' horses, crazily random lists of special abilities for monsters, and the strange way that bow ranges are calculated. Zubon's 'proud nail' list for City of Heroes focuses on the insanity of pitting high-level heroes primarily against a single villain group called the Carnival of Shadows. The Carnival has some serious issues, primarily stemming from their annoying attack moves and the famously weak amount of content at CoH's highest levels.
I think 'proud nail' is a really useful term ... can anyone else point out a proud nail from any other games that immediately spring to mind?