OTOH the way I look at it the PCs are experienced combatants, even at level 1. The players are looking at little lead figures on a grid and rolling dice. There are TONS of things that the characters would pick up on, and look for, that the players simply have neither the experience as actual combat experts nor the opportunity to pick up on unless the DM provides lots of description. Beyond that the DM isn't a combat expert either, and has not ever been in a lethal sword fight. He has no better idea of how to describe an orc that has about run out of luck than anyone else.
OTOH, what if a player rolls 3 14s and hits all 3 times. Does that tell him that the monster is an able opponent, or an inferior opponent?
So, it makes sense to provide information to the players that generally indicates what the characters would naturally be able to determine at a glance. Is an enemy close to defeat, is it an unusually quick (higher ref) opponent, or does its equipment or general construction make it especially difficult to effect with weapon attacks (higher AC). Given that the DM may not always know how to describe every one of these kinds of details narratively, and some of them don't really HAVE a single narrative explanation anyway, like HP, I've always felt that stating "oh, that guy looks to be in bad shape, you almost killed him." is pretty reasonable, and even saying, "you barely missed, his helmet deflected the shot" likewise.
Yeah, I don't buy it. We cannot say on one hand that the players and DM really don't have that real life combat experience, and then turn around and say that someone who did have that real life combat experience would know stuff at a glance. If we don't have that experience, how do we know that it doesn't take a few minutes to size up a foe for imaginary opponents in an imaginary world? Boxers typically do not come out swinging for the fences. They feel each other out, trying to find weaknesses.
This is one of those things that is reasonable based on how individual DMs want it to work. Personally, I would think that good opponents would sometimes try to fake out their foes, so they would purposely try stuff to make themselves look less capable than they really are. Lesser foes might try to make themselves look tougher.
If the PC is blind, is he still this combat omnipotent warrior? Any rationale can be used to support any POV here, but it does boil down to individual DM preference and how s/he prefers to run the game.
Btw, I do on occasion indicate that a PC barely missed (it glanced off his shield, etc.). But usually only if s/he missed by one (sometimes two). But not every time either.
I would also point out that the developers of 4e probably feel similarly, they DID after all describe half hit points as "bloodied" and I don't think that was accidental.
We actually used the "bloodied" concept of half hit points for years in 3.5 as a house rule. If bloodied, the PC is at -1 to all D20 rolls, -2 if 0 or below (for those PCs that can fight at zero hit points). We even had a stint were it was 2/3rds damaged for -2 and 1/3rd damaged for -1.