While I've employed "minions" of one sort or another in previous games, the idea that lower-level foes aren't a genuine threat seems to be an artifact of 4th Edition mechanics. In every other edition, I've been able to challenge PCs with hordes of lower-level foes. Since the party's defenses didn't consistently rachet up with their level, low-level enemies could potentially overwhelm them.
Been true since AT LEAST 2nd edition.
Taken from the black cover PHB.
Pg. 99, Full plate armour + shield = AC 0. While not achievable at 1st level due to monetary concerns, it most assuredly was by at least 5th level.
(look at the table for followers a fighter got and you'll notice that if a fighter had a 5th level fighter as leader of his followers, the minimum gear it came with was plate mail + shield. I personally used this as a minimum level or "expected" gear for fighters
keep in mind that if you look at the table of FOLLOWERS from the fighter excerpt, there was something very weird going on...For Elite units that the fighter gained as followers, one of the options was 10 mounted knights wearing field plate + shield)
THACO of kobolds, orc and bugbears 20, 19 and 17 respectively.
I certainly don't call being "effective" when you need to throw 20 kobolds just to get the possible of one hit.
Indeed...if you look through the Monstrous Manual, most monsters (60%) don't have a THACO better than in the teens...
Given that I didnt include magic armour, rings and/or dexterity bonuses, the idea that this is either a 3e or 4e problem I don't agree with.