So I've played a fair bit of 4E so far, but it mostly consisted of one playtest scenario at 13th level (in which everything being new was expected), and several sessions of Scalegloom Hall (which is almost entirely fighting kobolds). Last week I started running Keep on the Shadowfell, and the first scenario was fighting kobolds.
Last night was our second session. Another kobold encounter. Then an encounter with some goblins, a trap, and some rats. And here's where the cool thing happened.
The players knew at this point that different creatures (even different flavours of kobold) have different tricks up their sleeves--real tricks that can have a significant effect on how they fight and how they can be defeated. So even though they knew what a goblin was in general, they were genuinely worried about what the goblins might be able to do. The rats were even worse--in fact, the party barely survived the rats with little input from the goblins (who mostly held back to snipe from a distance). Despite a strong start to the encounter, in the end the heroes were forced to disengage, regroup, and rethink their tactics.
That's right: forced to retreat by some rats and goblins.
I love the sense of discovery that's coming from 4E--more than I recall from previous edition changes. 4E may have conceded a lot of "realism" (in the form of fiddly rules) in exchange for smoother gameplay, but it's added a lot of veracity through more sophisticated and variable bad guy tactics, more ebb and flow in the combat, and the higher level of challenge that can put PCs on the ropes in even a "routine" encounter.
I love the fact that players have to learn, through the encounter, how to deal with a challenge--they can't simply rely on their favourite tactics each time with faith that those will carry them through. (This is especially true at this stage, when everything is new--but will hopefully continue to be largely true.)
I'm surprised it took me this long to notice, but I guess that's because my previous experience was limited to a very narrow pool of creatures.