Personally, I didn't experience that vibe at all, but this is my thought on where that vibe might have come from:
1.) Expectation of mechanical balance: 3.X's "one system fits all" approach to PC, NPC, and monster creation (as well as the CR/EL system and wealth by level) created an expectation of "if its good enough for you, its good enough for me". By that, I mean monsters were built using the EXACT same tools PCs were (same bab/save progressions, feats 1/3 HD, skill points, spells used as special abilities). This meant that since (technically) the DM was using the same tools as the PC for creating monsters and foes, the DM was "cheating" if it didn't conform to those rules (If you met a kobold that breathed fire and exploded upon death, the first thing most PCs might wonder was: "What kind of Prc/Template/Feat was that?" rather than assume the DM added both powers and bumped in CR up 1).
2.) Everything is playabe: the ultimate expression of this mechanical balance; since EVERYTHING is mechanically balanced, that means EVERYTHING can be playable, right? The ECL/LA rules tried to discourage this, but they were still seen as an invitation to weird and exotic races being used a PC when they had NO BUSINESS being used as such (Savage Species, I'm looking at you). This was stretched out beyond logical conclusion when Dragons (Draconomicon), vampires (Libris Mortis), Mindflayers (Savage Species) and even Chain Devils (Planar Handbook) all got PC style write ups. If WotC things they can be playable...
3.) Prestige Du Jour: WotC glutted the market with PrCs at first, one for nearly any concept you could want. Alienist. Hunter of the Dead. Gnomish Nadkicker. Etc. Some were suitably generic (thief acrobat) but other good ideas got tied to either general flavor text (Nightsong Infiltrator) or very specific flavor text (Shadow Thief of Amn). PCs though wanted to take these PrCs because they powered up their character compared to core-only offerings, so DMs had to balance the specific, not-so-specific, and general PrCs and make them work in his game. And every new supplement added some new headache.
4.) Whats Good for the DM, Part 2: Since WotC require mechanical balance for all monsters and NPCs or the CR system went on strike, WotCs only way of expanding monster capacity was mechanically: templates, PrCs, feats, etc. Unfortunately, that meant those options were also available to some PCs, esp those who were playing monsters or monster-like races. Dragonborn of Bahamut don't seem too great, until you gave them metabreath feats. Likewise, no PC could qualify for Hulking Hurler until Goliath came about. Because of this, things that would be fine for an antagonist became fodder for PCs.
5.) But I want to use this: WotC sold books to players because the modular system in 3.5 encouraged it. New races, classes, PrCs, feats, spells, and magic items filled good room in a supplement and guaranteed a player might by an otherwise DM-heavy book (starting with the DMG and MM!) Naturally, if you by something, you don't want to let it sit on a shelf and collect dust, you want to USE IT. So let it begin the "Can I plays" from whatever new book came out.
6.) This Book Requires the... Late in WotC's development cycle, they started the system of occasionally referencing other books in supplements beyond the Core Three. This legitimized the idea that "anything is kosher" and you should mix-and-match as much as possible to make the best PC. PHB 2 had feats that targeted Complete Adventurer's Scout class. Complete Arcane had new spells for Complete Warrior's Hexblade. New Warlock Invocations appeared in Dragon Magic. PCs were encouraged to buy, and use, their books.
7.) Power Creep: Sad but true. Newer classes, races, feats, spells, etc were more powerful than the PHB offerings, and they subtly lured players to buy, and play, those weird ideas. Warblade. Warforged. Both creeped up the power on the otherwise classic dwarf fighter. DMs began to have a heck of a time knowing what their PCs were capable of.
