Frankly, I find them annoyingly untrustworthy. "Glorious" tells me nothing about Half-Life 2 outside of the fact that somebody apparently liked it. No, users, Assassin's Creed III is not a sandbox title, and "Where is Charles Lee" is neither clever nor informative. And as amusing as it is for those familiar with the title to hear Team Fortress 2 referred to as a "Hat Simulator" that is NOT helpful for people looking at the title for the first time, nor is it something that anyone in their right mind would do a search string for. Nor are "Shepherd." and "Wrex." helpful tags for Mass Effect. And for all that there is some mechanical similarity between Darksiders and Ocarina of Time, it is not a "Zelda" title, nor is it a puzzle game.
Point being that user immaturity had long since compromised the potential benefit of user-defined tags and Steam would probably do well to drop the system.