The opening cinematic is the shorter teaser that we've all seen, with the space marine vs eldar battle but not vs tyranids
You're forced to sign into Games for Windows Live if you want to play the Beta, which is kind of lame but whatever
You can play as any of the 4 races (space marines, orks, eldar, tyranids)
You select 1 commander, and they give a subscript to each indicating their specialty. For instance, as a tyranid I chose the Hive Tyrant, labeled as Offense, but I could have chosen the Ravener Alpha (tunneler) or Lictor alpha (ambush). Each race gets 3 commander types, and a list of their 4 abilities appears at the bottom of the screen while you're making your selection. The space marines all have 5 powers instead of 4, but some of them are very similar (and maybe weaker?).
Space Marines - Force Commander (Offense), Apothecary (Healer), Techmarine (Defense)
Orks - Warboss (Offense), Kommando Nob (Stealth), Mekboy (Support)
Eldar - Warlock (Offense), Warp Spider Exarch (Teleport), Farseer (Support)
Tyranids - Hive Tyrant (Offense), Ravener Alpha (tunneler), Lictor alpha (ambush)
The war paint selections are awesome, allowing you to pick any color from any canonical Warhammer 40k faction (I'm going with Hive Fleet Leviathan, the default)
There are only 5 maps in the beta, two 2player maps and three 6player maps. Six is the maximum allowed in a game, 3 on each side. I'm not sure if FFA is an available option. There are two gameplay modes, annihilate where you destroy the other opponent or victory points, where you have to capture and maintain victory points. This involves a sliding bar that starts in the middle and slides towards whichever player has more victory points at the time; if your score reaches 0 you lose (ie the bar has reached the other player)
The resources are requisition (as usual) and power, which must now also be captured. There are various power points throughout each map which generate power at a slow rate, but you can also spend requisition to build additional power generators on these points. Each race also has a third resource (Biomass for Tyranids, Zeal for Space Marines) acquired by slaying your opponents' creatures. This third resource is required to activate your commander's global abilities. For instance, all space marine commander types have the Orbital Bombardment ability, but it requires a lot of Zeal to activate, about 1.5x as much as a Terminator Drop Pod
My first game was pretty fun as the tyranids vs a techpriest , although I wasn't familiar with the units so I spent most of the game just screwing around and upgrading things. Your commander can receive up to 3 wargear upgrades (typically a weapon, armor, and some utility item) and most other units can receive at least one upgrade.
You can't build any buildings. You start with an HQ that produces all of your units but requires tech upgrades for the good stuff. There's a single defensive turrent, such as a heavy bolter, placed next to your HQ and that's that. Presumably defensive commanders can build additional defenses, but the game places much less emphasis on building. Of course, this could just be one of those missing things do to it being a beta
Graphically, this game shines. It looks fantastic and nicely polished
Overall, it's a pretty fun game to play but I'm really more of a base builder myself. Also, Zeal/Biomass/whatever takes a long time to build up, either that or the abilities are just too expensive. Also, you can tell the developers intended these games to be much more action-oriented by removing base building, but requisition/power practically trickles in. You can upgrade power sites, which is good because they're not especially common, but you can't upgrade requisition points at all; you just have to capture more of them. Space Marine Scouts might be cheap and disposable, but normal tactical squads cost twice as much. They're worth every penny in my opinion, but in the early game you're definitely going to want to blow a bunch of money on scouts, possibly upgrading them with combat shotguns. The slow resource trickle really forces you to build the cheaper units ASAP, not allowing you the luxury of teching up before building your army since you'll require a lot of power.
So yeah, the gameplay requires some polish, but it's a good time. If you can, download it and give it a try. Even if you don't feel like playing against others, you can always just set yourself up with some computer opponents and screw around with the different races.