In our visit to Valve as part of the 1UP Cross Country Game Tour, writer Erik Wolpaw detailed the idea behind the spite-your-neighbor game mode, and why it just didn't work out.

"Along with co-op, [we had] the idea of sort of a competitive Portal multiplayer," he said. "We went down that path, actually, for a little while and had something up and running -- the best way to describe it is sort of speedball meets Portal. You know, a sports analog. And it quickly became apparent that while it's fun for about two seconds to drop portals under people and things like that, it quickly just devolves into pure chaos. It lost a lot of the stuff that was really entertaining about Portal, which was puzzle-solving. Cooperative puzzle-solving was just a much more rewarding path."

The actual gameplay of the mode involved moving a ball from one end of a space to the other quickly using portals. The competition would try to stop the player by, for example, trapping them with a portal shot onto the ground. Wolpaw says in practice "it was a hot mess."