The alpha footage they showed (from WiseUp216's link above) seems alright to me. It's nothing we never saw from previous similar (rather, identical) games, but it's nothing we don't hate I suppose? I'm rather neutral about it for now. I'm not disliking what I see, but watching it doesn't scream "OMG! I'll definitely have to play that mode too!". I find the 'demon rune' mechanic to be somewhat interesting, it reminds me a bit of Battlefront's hero units (and that's probably the point, but instead it's demons of course).
I think that I would have preferred it, however, if the demon you spawn as would not stay in the team you were playing as, and would only serve a purpose of match 'disruption' on both teams (or, well, anyone of any of the two teams whom happens to be there when you are, as a demon that is). The fact that the demon remains in the same team will result in a 'race' to get it (probably) since it seems quite powerful and could make a difference between victory (from a near loss position) to hummmm... to complete slaughter? We'll see...
I'll probably (surely) stick to UT if I want my arena-style shooter fix. But I'm not saying that DOOM's alpha multi-player looks "bad" (I mean not just visually, also game-play wise). They still have months of testing/balancing/polishing/fixing/adding content so it might only improve from its current status.