Coming hand in hand with these legendary items are the issues of imbalance that they cause. In PvE, terminology has started to crop up that puts legendary and non-legendary DPS into two separate categories of competitiveness. Concepts exist such as "enhancement shaman are currently one of the top non-legendary DPS specs..." -- a category that encompasses only 10 of the 22 DPS classes in the game. In PvP, concerns about burst damage have arisen, which was a big factor in the nerf to DTR that came in 4.3.
The issues with legendaries
Legendaries cause a balance problem, and that's a problem that's been exacerbated by two things in Cataclysm: a horrifically wide spread of specs that can use them, with 12 specs or five classes in total having access to legendary weapons right now, and incredible ease of access to legendaries (for the heroic raider).