For me, i point out the NPCs. i usually even buy their items based on the guidelines, but often will shift this either way for impact.
One of the benefits the classes provide is giving the players a 'familiarity with the world. The classes represent the common archtypes, the way powers are related, and give the players an understanding.
Whenever something breaks those ruoles, it should be a clue, a noticeable moment, a point of interest and not just "well Billy was rushed again" thing, IMO.
Hence i tend to work up the noteable NPCs, even the miscellaneous NPCs have class-level although much briefer writeups.
This is particularly true of attributes... as its tough to get too many high attribute scores.
looking at your examples...
"- city guard, attack +5, Power Attack, Spot +10, 20 hp, AC 20"
Ok looking at this as a GM i have to know where their AC 20 came from, so as to resolve touch attacks and flatfooteds. Just Ac20 is not enough info anymore.
Looking as a player, i would be wondering how he got a +10 spot to oppose my hide/sneak attampets... especially given he had no sneak attack or no major hp totals to indicate high levels. Assuming a 14 wisdom, he has to be 10th level as a fighter to be this good. If multiclassed as a ranger he still needs more levels than 20 hp would justify.
See, in my game a lowly guard grunt spotting the thief with a +10 spot would be seen as a clue, probably noticed only after more than a few guards were able to notice the thief. The PCs would start to wonder whats up with these guards, and probably find it significant enoguh to investigate.
By just making numbers up out of the blue and not worrying about whether they make sense, you lose this.
"- orcish assassin, attack +10, all thief skills +15, sneak attack +5d6, 50 hp"
the sneak attack weigh him in at about 9th minimum. the hp says con bonus, all thief skills however covers a lot of skills and even with the thief getting 8 this would require a high int... and a dex bonus and an int bonus... so now we have a guy with str bonus, dex bonus, int bonus, con bonus... and we still dont know what "thief skills" means... does it include spot, listen jump balance?
This one would likely as not avoid notice since most of the levels seem apropos, and in all likelihood most of the thieves skills would not be used so the PCs do not know how good he is at so many things, but if i were to actaully use the bulk of those thief skills i would start to wonder myself. i find its too easy to use NPCs "dully" when i give them openeneded lists of abilities. its much moreinteresting when the NPC dont have "everything i need" and thus have to improvise as well as the PCs.
"- archwizard, 20th level, attack +15, all knowledge skills at +10, Spellcraft +25, Concentration +25, 100 hp, all metamagic feats, pick spells as needed from the list"
Especially for wizards and sors, i point them out. Spellcasters in particular. Allowing these guys "unlimited spells" or picking their spells on the fly grossly distorts their abilities. my NPC wizards often walk around, just as the players do, with some open slots, so if the PCs gain surprise, they wont be meeting a full strength geared for offense wizard. My NPCs often have a spell slot or two here and there devoted to non-combat interests. In short, their spells are appropriate to their situation.
As for feats, feats make and define a great many characters, especially mages, as such any sort of a wildcard "all metamagics" is out of the question. besides, i think there are more metamagic feats than the 20th has slots.
a far better approach is to spend an afternoon pointing out some base characters... grunt fight, thug, elite archer, novice sorcerer, priestly wanderer, etc... do them right, and use them as a template.
If you decide you need a thug boss, take the thug template and add a few fighter levels on the fly... thats pretty easy... and even if its not exact it will be close enough.
