well, i've read through KotS a few times now before running, and so my first impressions are
1. the double page encounter spread is awesome. finally, no more flicking around a book to find creature stats, maps, treasure, tactics etc.
2. monster entries are easy to read; no more massive stat blocks with teeny tiny writing
3. the structure is very good and easy to follow;i often found the old 3.5 published adventures a bit of a pain to read and avoided them after the first one i read. this one has a very straightforward "fight-interlude-fight" structure
4. There are quite a few spelling mistakes, and random mathematical errors, particularly with the pregens, which i am not impressed with. its a minor issue, but i really wish they would proofread something to death before releasing it.
5. Finally, alot of people have mentioned the quality of the paper of the books and in an earlier post there was a comment on the art. I think whilst the paper is thin, its not like the toilet paper i expected from reviews on these boards. As for the art, it beats the snot out of what i've seen in the old 3.5 adventures. One thing i would have liked is for small picture cards of the monsters that the PCs are going to fight, so you can put that on the table as an initiative marker and cut time from describing monsters. (The picture on the cover for Kalarel is much, much cooler than the picture in the book too, which looks v dorky).
In all, i believe one of the major aims for 4e was to make DMing easier and more fun. As a DM who spent AGES on prep, i think this is a promising sign of things to come; if WotC can just work on their proofreading they wont have to spend time doing following "FAQ about KoTS - what we missed/messed up".