I'm playing in Icewind Dale II and noticed, that armour class of characters is positive, while in AD&D 2 it was negative. Is this one of the new additions of AD&D 3.5, on rules of which ID 2 based?

IM not 100 percent sure but it has both positiva and negative armour class-the lee the better

It's the 3rd edition rule...
the point is if let's say the character has 14 (10base+4armor) you have to roll a d20(+ your bonuses) and if the rolled number with added bonuses is greater than the AC, it's a hit...
No more THAC0

The 2nd edition AC rules were terrible, and confusing for non-/new players. The 3rd edition cumulative AC rules make much more sense: the higher the AC number, the higher your attack roll has to be to get through.

Yeah, they stripped down a lot of the clutter for 3rd Edition, but still made monks a completely overpowering class. I don't know if that was changed in 3.5, but I don't really care. I love monks.

It's the 3rd edition rule...
the point is if let's say the character has 14 (10base+4armor) you have to roll a d20(+ your bonuses) and if the rolled number with added bonuses is greater than the AC, it's a hit...
No more THAC0 [/b][/quote]
What about impossible throws (when armour class 20+)? It is still 20 is hit and 1 - miss?

Yes it is...

20+ isn't impossible. You apply stats modifiers and attack bonuses to your rolls, plus the BAB for your class. 20 can end up being considered quite low for an AC

In the old versions (which I used to play), both positive and negative AC were possible. In fact, the lower you got the better it was. It was also possible to have an ac of 0, which was not bad.

I still prefer version 2.0 though it has so many choices and isn't as limiting as ver 3.0 and 3.5