What aspects of a game are not protected under copyright?
Fundamentally, the OGL exists to (1) allow D&amp;D 3.0 supplements to be made by 3rd parties and (2) to ensure the 3.0 Dev Team could take their mechanics with them if they got fired.

(1) can be seen in the explosion of material in 2000-2002.(2) can be seen clearly in Pathfinder and Arcana Unearthed.

While mechanics can't be copyrighted, and the US Patent Office almost never¹ grants patents on game mechanics, the Open Gaming License provides a means of reusing the literal text, and is a two-way license. Any mechanics you don't declare as product identity are likewise open. 

This sharing could have been bidirectional with Wizards, but they appear to have decided not to go that route in the 3.x era.

The ability to use the literal text, combined with the lack of a formal OGL mark², only the D20 STL one, at the time of inception, allowed D&amp;D-compatible d20 System games to flourish; they all "required" the D&amp;D core rules³, and thus helped drive sales.

In general, it made D&amp;D 3E one of the most used game systems in print... at least until Wizards moved on to 4E. And the D20 SRD is still one of the most used reference documents in the open gaming movement; D&amp;D 4E no longer is part of that. (It will be interesting to see how 5E fares, and if it uses the OGL or not.)



Sidebar: Fonts

ML-111 specifies that typefaces are not copyrightable in the US unless individual characters constitute recognizable works of art.

ML-443 reads, in part:


  Pursuant to Congress's judgement in the 1976 act and case law, the Copyright Office does not regiser claims to copyright in typeface designs as such, whether generated by a computer program, or represented in drawings, hard metal type, or any other form.


It does note that the program to generate a picture might be copyrighted, but the font itself can't be even if the font-as-program is. 

The earlier  ML-393 notes that font-as-program is unregisterable as the font itself is in fact not copyrightable, with the exception of fonts that are comprised of recognizable artwork. 

It should be noted that the particular "look and feel" is a viable and defensible trademark, and for games, Fonts are part of that look and feel.



This isn't legal advice; for legal advice, contact an attorney at law licensed for your jurisdiction.



Notes

¹: The exceptions are few and thus highly notable. Wizards' patent on "Tapping" in card games being the most notable of the lot.
²: The OGL mark is a later add-on; Wizards only released the d20 system logo and a license for it requiring one to not include certain mechanics...
³: not that most of them can't be played without them. The missing content issue was solved in several d20 games by simply publishing an OGL-only web-enhancement with the missing rules.