I don't think the atmosphere is really any better, just because the sky's dark doesn't make it scary for me, especially because it's not really that dark, you can still see just fine outside of tunnels/sewers in both games.
The point about no "movie moments" is completely irrelevant. It's going to be random, but they're just as common in L4D2 as in L4D1.
With the characters I don't think it's greatly impactful. The only character who I don't find greatly interesting is Rochelle, all the 7 others are good. Ellis is better than any other character, with his crazy stories and general insanity.
I never noticed more or less bugs in L4D2 than in L4D1.
I largely like the new infected. If you think the charger is similar to the hunter, you've never been in a tunnel or corridor or just been in a line or bunched up with other people. Charging 4 people then having someone spit on them in Versus is a vastly different mechanic than having 1 person get pounced, having someone spit on them, the hunter getting instantly killed and 1 person taking minor damage rather than 4 people taking quite major damage. The spitter is my favorite infected of all, even if it is a bit reliant upon other special infected doing something. I find the jockey fine in campaign, although only useful in Realism Versus, not regular Versus.
As for little touches, I've noticed neither being better than the other.
So essentially I find them the same, just L4D2 having more special infected, much more variety with weapons and better graphics. The only legitimate complaint I think is that it's a bit similar to L4D1 for a sequel, although Valve's support is very good with having a new campaign and mutations (for PC anyway, given that it's free).