Having loved playing L4d2 and played hundreds of hours especially in the dedicated servers my view on the beta was quite impressive, bearing in mind that most developers do not give out a complete 1 act within an Beta normally you just get 1 map. Graphically very impressive there was some things they could improve on (water reflections / mirror reflections) they looked blocky on the reflections but that could just be limitations of the beta. Some weapons felt underwhelming for example the m16 (burst fire really!). I loved the idea of being able to upgrade the weapon for coins but my complaint with other friends is that crates within the map is first come first serve (people don't even look if they need to upgrade the weapons and just take the upgrades). so i did ask if they can make it so attachable can be dropped really quite pointless having a full scope on a Ump where someone who has a carbine/sniper needs it.
I love the new specials and you have to think tactically so it really encourages team play and thinking about the next area, however i think the pet peeve is after 3 fails you have to start the whole act again until you reach the checkpoint. especially if the corruption cards are really bad in Nightmare they really punish you, the original L4D2 if you failed the mission you would restart back at the safe house so i think they missed the plot on this one if you are trying out the nightmare mode. The cards itself looked good on the tin until you try harder difficulty 1 CARD for nightmare at the beginning on act 1 but 3/4 corruption cards that is not balanced, with 250 coins so you are limited to not buying any upgrades just about buy health and pipebomb.
Bugs, my friends experienced some CTD and ADS (Aim Down Sight) bug where you couldn't aim so had to restart the server, some NPC lagging out or weapons freezing in mid air. but that was more funny than anything.
If they can fix the balancing of the game and weapons, and graphical issues it could be really good game. However i do wonder how many acts there are because if it's quite small acts then this could affect replay ability.