Hearing this from someone who has actually played the demo is surprisingly exciting. I for one am definitely looking forward to getting Mass Effect 3 myself and playing it, especially since I found out that Quarians may be a playable race for multiplayer. I'm fully expecting it to be just as mediocre as you mentioned, but it might be a nice escape from playing the same single player missions over and over again while still wanting to play Mass Effect 3.
As for the points you outlined on the gameplay, I am severely hoping for plenty of variety when it comes to the modifications you can add to your weapons, particularly laser-sights or scopes for the assault rifles. If I can, I would definitely modify a Mattock into even more of a cross-breed between a sniper and assault rifle, with a scope, longer barrel and more shots-per-clip.
The variety of enemies is also very intriguing, seeing as, unfortunately, Mass Effect 2 was rather poor when it came to different enemies to fight. That didn't mean it wasn't a fun game, but it sounds like the third installment is taking the potential the second game had, and is greatly expanding the universe, not just for gameplay, but hopefully even lore.
EDIT: Saw the latest Mass Effect 3 trailer. Quarians are CONFIRMED for Co-op!