In a lot of discussions, players of older editions will profess the concept that 1st-level PC's are basically just starting out, raw, inexperienced (literally!) and basically only a smidge more powerful than the average farmhand.
I always thought the "zero" in TSR-era D&D/AD&D was...
Preemptive disclaimer: As always, unless otherwise stated, all of this is 'In my opinion'/'It is my position that...'
Like most things about the olden days, exactly what the zero-to-hero concept really means is not entirely consistent. I think the overall premise, at its core, stems from PCs starting out as fragile entities that can drop from 1-2 successful hits from the creatures they will be facing -- and gradually become wildly more powerful in terms of staying power, action options available, and opponents regularly taken-on. All the rest is negotiable/people had wildly different takes on.
*which the monster manual telegraphs to be 'generic enemy soldier'.
Some people always envisioned their characters as (say) farm kids tying to escape a life of monotony to make it rich or die trying. That conception might have the first dungeon being the first time the fighter character strapped on their armor. Others thought of their characters as semi-seasoned soldiers, thieves, etc. who just decided to adventure instead. There's certainly enough evidence in the pre-2E books to support either position -- be that level title of veteran or an oD&D fighting man being effectively a base soldier unit to AD&D's 0th level fighters being potentially most (even seasoned/non-green) soldiers to the starting age roll to whatever else considered important. Everyone envisioned their characters differently, and could find something in the text to support it.
So is this the moment where the definition of what a low-level character is changed?
I would say no. What The Complete Fighter's Handbook* did was to give the PCs
background. It codified that what your character did before becoming a PC mattered, if even just a little bit. Mind you, this was a style of play that existed effectively since the beginning. However, as far as book support, it was near the first.
*and I would posit that the 2E PHB did before it, with the secondary skills section. Possibly before that with 1E non-weapon proficiencies in Oriental Adventures and the Wilderness/Dungioneer's Guides.
I don't know (and I used the Complete books for a bit in 1989 - 1991) how much they actually changed the power curve, but I do remember them complicating character creation and adding to the general feeling of system bloat - though that was usually considered cool in the late 80's and 90's.
I don't feel like the changes in the
Complete series* stepped up the power curve by an amount unheard of in the system already. I feel like which attribute-determination methods the DM allowed had a bigger impact. Even the much-discussed Bladesinger kit from
Complete Book of Elves was ballyhooed mostly because it was
strictly-
better than regular fighter-mages**, not because it was overall OP. A few later ones like
Warriors and Priests of the Realms had some selections which gave specialty priests weapon specialization or fighter Str/Con benefits without negatives, but even those mostly help alongside good stats.
*Players Option series, on the other hand...
**not actually that powerful, given 2E's armor-casting restrictions
Complete fighter gives (of note):
- The katana (and longspear) -- nice if DM changes magic item charts.
- Weapon proficiency groups (highly beneficial for flavor, but for optimization everyone would still pick bow, lance and long- or 2H sword).
- An alternate route to penalty-free two-weapon-fighting (the real power boost of the edition)
- Shield-bashing: 2wf with a shield -- useful if you have magic shields to make shields worthwhile (but still need DM house-rule to have shields be able to hit monster needing +X weapons)
- First printing had weapon specialization for paladins and rangers (changed back in next printing)
- Various kits that give an odd bonus proficiency or two.
Now, moving the power curve
downward, the
Complete series did that plenty of times. Be it specialty priests from
Complete Priest, savage characters using bone or stone weapons, historic campaigns where magic was absent or limited, or swashbucklers/pirates that specialized in little or no armor and mechanically inferior rapiers/sabres/cutlasses. All of these down-powered characters to enhance flavor.