Beast runs on a currency system -- the points you get for killing enemies and assisting your teammates is effectively what you use to buy different Locust units to play as. In the beginning, you only have access to Tickers and Wretches, but as the waves progress and you earn more, you can spawn as Grenadiers or Berserkers or even the Armored Kantus. It should be noted that in the demo we played only certain Locust units were selectable during certain waves. I liked using the Grenadier, but wasn't able to select that unit on each and every wave once I accrued enough tokens to buy him.
The map I played on was called Mercy and featured plenty of tight corridors and barbed wire, which slowed down my advances and gave the humans plenty of bottleneck points. It was a mid-sized map, offering a few vantage points at the top of a symmetrical staircase where the humans could overlook the courtyard and lay down suppressing fire -- and, believe me, the CPU players took every opportunity to do that. This meant that cooperation was key to survival.
So one of my teammates would spawn as a Ticker and rush a cluster of human soldiers, distracting them long enough for me to either flank around behind them and lay down my own fire or lob grenades at them. While cooperation is nothing new to the Gears of War series, all the variations in unit abilities really made for a fresh experience. It's satisfying when an enemy is focusing on somebody else and you emerge from their blind spot to make murders.
The demo only lasted four waves, so there's no telling on how long this game mode will last or what surprises are in store for your Locust squad as you progress.