Who remembers the !hatme command from the funhouse, and availability of hats to the clan members. It wasn't even just the "official" hats, but other fun ones like bunny ears, road cones, cowboy hat, sombrero, etc.
With one of the very recent TF2 updates, Valve has made changes to purposely (according to Robin Walker himself) hinder any 3rd-party player attachment, such as hats (or any random model, not even on head) and even weapons that the player does not own.
Needless to say, I, as well as many other server admins, as quite disappointed. Many players are as well whom are regulars on servers that run these type of mods. It has even broken certain custom gameplay modes. Prophunt was broken until they later released a fix for it, and pyro dodgeball still is.
Valve has went from a the-sky-is-the-limit approach with modding, to doing the equivalent of putting restrictive DRM on the client. I refer it is as DRM as warnings end up printing in the server console regarding trying to equip an item that one does not own. This comes right after a microtransaction interface has been found in the latest Steam API. (hats for $$$, anyone?).
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Some redundant links for further reading/discussion