I'd like to propose a few more specific buzzwords:Benimoto said:I never saw the "Christmas Tree Effect" as a big problem in 3rd edition, because to be honest, I was never totally clear what it was. It seems to me that it's more of a very loosely defined buzzword than an specific phenomenon. There's been like 4-5 competing definitions in this thread already, ranging from "PCs have an item in every slot", to "PCs need +X items to be viable" to just "magic items should be rarer and more special... Christmas Tree!"
"Layer Cake": The individual layers of a layer cake aren't all that high, but when you keep stacking them up, you can build something ridiculously tall. Similarly, you might not have a problem with a suit of +5 armor, or with a +5 amulet of natural armor, or with a +5 ring of deflection. However, when a PC uses all of them together, it can cause problems, because, like the layer cake, it builds up to an unreasonable height.
"Sega CD 32X": There were a few games released for Sega Genesis that required both the Sega CD and Sega 32X expansions in order to play. The resulting system was a ridiculous behemoth with more things plugged into it (32X sticking out of the top of the Genesis, SegaCD on the side, extra power cords, extra video cords) than a person on life support. Thus, I propose the 'Sega CD 32X' effect to be the one where PCs feel the need to put an item in every slot just to stay competitive.
"The Incredibles"*: One line from the villain of the Incredibles was that "when everyone is special, no one is". If encounters assume that the PCs all have a certain level of +X items, then the DM finds himself in the position of having to choose between letting his PCs be severely underpowered, or hacking his way around the rules, or just letting them have the items - and then players don't feel like a +5 longsword is 'special' at all, because everybody has one.
*I don't mean to claim that this idea is something new to the Incredibles; quite the contrary. However, this iteration of the basic idea is recent, succinct, and pop culture.