Where that Old School Feeling is Important

Feeling does come into play when you are trying to SELL products to the Old School markets. Because the experience of the game is very much dependent on which rule set you use. The Old School market has currently settled on rule systems that take their core concepts from a selection of games developed in the 70s and early 90s. I wouldn't create a dungeon and try to market it using the full D20 rules to this market no matter how much it adheres to the principles in Old School Primer. I would pick OSRIC, Swords & Wizardry, Labyrinth Lord, or one of the other rule sets out there.

Remember that the Old School market isn't a narrow niche of dungeons and old rulesets. Games like Encounter Critical and Mutant Future are considered part of the Old School market. The former because it looks like it ought been published in the late 70s and the latter because many perceive it as Gamma World done right and it is based on the Labyrinth Lord rules.

In terms of settings it was the original 1974 rules that spawned Barker's Empire of Petal Throne. Carcosa shows there are still people thinking of presenting unique settings using the framework of the 1974 rules. This the area where my own interest lies with my Points of Light series, Wild North, and Majestic Wilderlands.

There is a lot of room to creativity within the Old School Market. However you have to take its personal preferences into account. For example in adapting the Majestic Wilderlands for one of the old school rule systems I made sure that I stayed within the spirit of the rule system. The result isn't quite like how a character is created in GURPS for the Majestic Wilderlands but the point isn't to make OD&D work like GURPS but present the Majestic Wilderlands in OD&D terms. It helped that GURPS 4th edition went into templates in a major way and so did I.

So if you are planning a product for the Old School market plan on using one of the older rulesets. Don't try to market your product as Old School in feel and use Exalted, GURPS, etc as the rules. You will wind up appealing fans of that system and not the Old School market as a whole. You can get away with using D20 as long as you limit yourself to what in the older rules and limit your stat blocks.