...I do have to take issue with the suggestion of a master index.
There's a definite line, IMO, between where it is a designer's responsibility to balance feats/spells/etc against each other (if not purposefully ignoring balance) and where it is a DM's job to balance.
When a product is designed, the only balance issues that the designer needs to worry about, IMO, is if the options in the book are balanced against each other. It's certainly helpful if the product is internally consistent with other products in the same line, and similarly helpful if it's balanced against the PHB.
I'm not complaining about internal/external balance so much as redundancy. If you're designing a D20 product, its supposed to be compatible with either 3.5Ed or D20 Modern. If you ignore that aspect when designing your product, you're going to cause problems for the end user.
One of the ways the game maintains its balance is by making feats that offer a particular kind of benefit obtainable only once per PC. Feats that offer similar or identical benefits create nothing but problems. To be fair, I don't think that there are designers out there looking to create broken feat combos- such combinations typically occur because of a lack of awareness of the other feats- as the saying goes, the left hand doesn't know what the right hand is doing.
If, as you posit, someone designed a feat for their product that would increase the crit threat range of a weapon by a certain amount based upon a PC's BAB, for instance, but ignored the existence of the PHB's Improved Critical feat, it wouldn't matter if the feat is otherwise balanced with the other feats in the product- the first time a PC combos that Feat with IC, the DM has a problem.
Can the DM rectify this? Certainly- he can bar one or the other, or make some other kind of ruling...but he shouldn't have to.
Hence, the utility of a Master Feat Index.
Furthermore, I'd argue that redundant feats are a loss to the purchaser. If you publish a book of feats that has "200 New Feats!" but 30 of them have been published in substantially identical forms by other publishers, those who have purchased those other works have really only gotten 170 new feats- you've shorted them.
Again, the Master Feat Index proves to be a boon.
However, these are not absolute requirements - supposed the whole purpose of the supplement is to bump up the power level a bit? There's absolutely nothing wrong with that, as long as it's known that's what the product is doing.
I agree.
Additionally, designers are not omniscient. There is no possible way for them to anticipate the interactions of what they have designed with what they haven't (except the PHB).
First, I'd argue that if you're designing something to be compatible with a WotC game, you should be intimately familiar with the core material, and at least somewhat familiar with their accessory products to make sure you're not 1) reinventing the wheel or 2) creating a truly bad combo.
Second, a Master Feat Index would go a long way towards making it easier for designers to anticipate interactions with both WotC and 3rd party products.
This is the DM's job - to make decisions about what is acceptable and what isn't in their game. If something is silly, if a product is overpowered, or if there's simply a preponderance of feats, the DM needs to step in and make a judgement.
Once again, a Master Feat Index would go a long way towards making it easier for DMs make those inclusion/exclusion decisions.