(Can you help? Has the situation got better or worse with versions 4.x?)
Problem: no texture compression could be specified
The patented S3TC algorithm was considered for inclusion into this standard, but the OpenGL ARB working group decided to not include it shortly before the release of the OpenGL 3 specification. Since S3TC is the only texture compression algorithm implemented by virtually all of today's graphics hardware, OpenGL does not (unlike its competitor Direct3D) offer a texture compression system for RGB or RGBA images.
While not required by the OpenGL standard, many OpenGL implementations can or do incorporate the OpenGL extension for S3TC, EXT_texture_compression_s3tc. Without it they have severely degraded performance/quality.
Problems for Mesa developers
The developers of Mesa have also encountered patent problems with "Floating point textures and render targets (in core GL3), certain compressed texture formats ".
Microsoft's 2002 acquisition of OpenGL patents
Microsoft believes they have patent rights relating to the ARB_vertex_program extension. They did not contribute to the extension, but are trying to be upfront about it. They're offering to license their IP under reasonable and nondiscriminatory terms; will license rights to the extent necessary, provided a reciprocal license is granted to MS. Granted on 1:1 basis for OpenGL 1.3, 1.4, and earlier versions. 
This raised concern among free software developers because "reasonable and non-discriminatory" ("RAND") terms can include royalty payments, and because depending on the good will of Microsoft might cause problems in the future.
Possible purchase of these patents by OIN
- (see also: Microsoft sells patents to OIN, 2009)
In 2009, Open Invention Network bought some or all of these patents from Microsoft.
Related pages on ESP Wiki
- Harm to standards
- 3d graphics patents
- How to avoid a specific software patent (hint: doesn't really work)
- The specification for EXT_texture_compression_s3tc, see "IP Status"
- Mesa FAQ 4.3 explains that patents are blocking full implementation "We've been unsucessful in getting a response from S3 (or whoever owns the IP nowadays) to indicate whether or not an open source project can implement the extension (specifically the compression/decompression algorithms)."
- freedesktop.org's page on S3TC
- Another project mentions fear of patent issues
- SGI transfers 3D graphics patents to MS, The Register, July 2002
- Microsoft: we own patents on OpenGL, ZDNet, 2002
- FWIW, a search for "opengl3" in the freepatentsonline.com database yields 3,500 hits, but there's nothing to indicate if any of these are necessary for an implementation of OpenGL