The D20 logo is fine, it is not the logo that matters. It is the requirements to place that logo on your product that matter.
For example, before WOTC changed the decency standards we had products from Mongoose with nudity and gore, some companies published real world settings with real world politics and religion, and of course the notorious Book of Erotic Fantasy came out. When they changed those standards, those books could no longer claim compatibilty with D&D. In fact, they cut the BoEF off at the pass if I remember correctly and it had its own d20 logo and wording saying it was compatible with the 3rd edition of the world's leading fantasy role playing game etc etc...
But there is another area that the D20 logo didn't do a good job covering. That was how well the game material was designed and the publishing standards of the book.
All WOTC has to do is add quality control that places the same standards on publishers of 3rd party materials that they place on themselves. Those 3rd party companies in turn should get access to the design guides and required standards that their products have to meet, and in turn they get to put the d20 logo and say they are for use with D&D 4e. Simple.
It is this second half of the logo that WOTC never really took care of. The part that controls the qualitiy of presentation and the crunch. In turn, we got amature looking art and graphics or poor designed mechanics.
If you want the D20 logo to mean something it needs:
Decency standards, prohibited subject matters (i.e. real world religion), presentation requirments, and assurance that the mechanics were built with a design package beyond the SRD (i.e. the style bibles that WOTCs team uses).
This is why I think companies should have to pay a liscense fee for the D20 logo. It seperates the amatures from the pros and makes the company take the add-on to D&D serious.