Wildly radical, myself.
Anything in the three core books/SRD is "acceptable." Magic of Faerun is acceptable in part. All other WotC products to this point (though UA may become the notable exception) are "Right Out" - because everything in them has, more or less, been done better (IMO) by 3rd party publishers!
I probably use more "3rd party" material by a 5:1 margin than WotC material.
Of course, UA appears to have quite a bit of good stuff in it, so I may allow it in as well, which will cut down the 5:1 margin some.
Specific items I think have been truly inspired/brilliant strokes from 3rd-party publishers:
1.) Second World Simulations' Master of Arms - The way "combination moves" are handled.
2.) Monte Cook's Arcana Unearthed - The spell preparation technique - spontaneous casting like a sorcerer, but your "known spells" can be shifted daily.
3.) Mongoose's Quintessential Fighter (revision) and Encyclopedia Arcane: Elementalism - "Demilevels" - rather than full-fledged character classes, you have a 5-level "Semiclass" for which XP is directly expended to gain extra special abilities (call them "Fighting Styles" in QF, "Circles" in EA:E). This was independently developed by several other publishers nearly simultaneously, but IIRC, Mongoose published them first.
4.) FFG's Spells & Spellcraft - Adding special abilities to your familiar (also appears in their $4.95 Masterwork Collection). A MUST for anyone who has a familiar
5.) FFG's Path of... Series - Legendary Classes and the associated Legendary Abilities (one ability per level, the ability's "power" is tied to the level when you originally take it)
6.) Thunderhead/Mystic Eye Games - "Adjustment Boxes" to adjust/scale encounters to handle a party of a different makeup than Fighter/Wizard/Cleric/Rogue.
7.) Malhavoc Press' Book of Eldritch Might - Not for the content, per se, but for basically creating the electronic publishing industry (which has had some absolute gems in its own right).
8.) Sword & Sorcery Studios' Relics & Rituals - Rules for Ritual Magic
9.) Green Ronin Studios' Hammer & Helm - First book of which I know that introduced the concept of Bloodlines/Feats that grant special powers.
10.) FFG's Mythic Races - First book of which I know that included something that very much resembles "ECL adjustments" as we know them today.
I'm not saying every one of these products is universally fantastic in its own right, but I think these 10 ideas I noted above were gigantic steps forward for RPGs and certainly improvements on aspects of the "Core" d20 system. These all get mention for doing things "first" or simply "better." To limit oneself to WotC only really limits your creative avenues, as many of the ideas in Unearthed Arcana are essentially derived from/inspired by the above works.
Everyone else will have their own favorite 3rd-party bits, I'm sure, but I would suggest that you show these 10 "bits" (well, 9, I guess... #7 is a bit dodgy as "rules" go). Compare them to the rules (or lack thereof) for similar things from WotC (note publication dates where these are "Firsts" instead of "Betters"). If that doesn't convince someone that WotC is not, cannot, and should not be the "End All Be All" of game design companies, well, there's really not much more you can do.
--The Sigil