That looks much better than the crap YT video. The art style looks pretty sweet. After watching that playthrough I'm guess they made the characters overly powerful in order to show them moving along through the large seamless environments instead of getting stuck at the same spot for a long time?
I've been playing the Alpha for a while now, so if you guys have an Q's just ask!
Regarding levels of enemies - The areas you spawn into are huge, with many different sub-areas around, and many tunnels / caves connecting everything, as you travel around, enemies are constantly spawned, at various levels, relative to your own level. So if you're lvel 6, sometimes you'll see a group of lvl 4 enemies, and you can mow through em, other times it's level 8's, and they really kick your ass. The boss battles tend to be very hard, requiring good cooperation between all 3 players (if everyone dies, it's a painful checkpoint restart, same as co-op halo. If one of you stays alive, the others will be respawned aftyer 30s).
In addition to boss battles, you get random "Public Events", where some big enemy, or multiple groups of enemies, will appear and multiple players can choose to join in, or not. These are also very hard, and you achieve a rank, from Bronze > Gold, depending on whether you fully complete the objective in the given time. I've done probably 6 special events, only getting gold once. It's hard to get gold, but it's mainly a time issue rather than pure difficulty (gotta kill everyone in 5 minutes, for example).
You gain loot after defeating bosses, special events, or finding chests. After beating a boss, the players are ranked, and it appears your loot drop is related to how well you performed. In special events, it seems that loot is related to whether you got Bronze, Silver or Gold.
Quite a few sections of the Alpha have been essentially locked off, by enemies that have infinite health, and can not be destroyed. So there's a huge variety right now, in challenge level, as you move through each map.
As far as I can tell, multiplayer uses your existing equipment stats from the main game. So it's super important to have good armor (yes, you have a full armor set), and great/unique weapons, if you want to stand a chance. They say that there's some level-balancing going on, but it's hard to say what it is.
The way they handle Ammo is really cool. There's 3 gun slots that can be switched in realtime, Normal (BattleRifle/DMG), Special (Shotgun/Sniper) and Heavy (Rockets / Machine Gun). Each ammo has a unique color, White/Green/Purple. White Ammo is plentiful, while Green is less so, and purple is very rare. So you always start out with your basic weapon, and essentially 'unlock' the others as you find ammo. I tend to keep my Heavy ammo saved for a big special event, or really tough group of enemies.