The developer has added an "onslaught" mode with a tweaked AI Director that dynamically generates swarms of aliens based on various factors, including the squad's stress level. This sounds a bit like the experience Left 4 Dead provides and it can be enabled on any difficulty so "no mission plays the same way twice."
There's also a new "brutal" difficulty setting, which is apparently no cakewalk, because nobody on the Alien Swarm team has managed to beat a mission on it. If that's still not hard enough, you can toggle "hardcore friendly fire," which increases the damage of friendly fire, forcing players to coordinate their movements.
To reward dedicated players, the game has also received three new IAF Promotion Ranks: Platinum Star, Osmium Star, and Iridium Medallion.