Simplest example, how does an Avatar from one proprietary system travel to another? You would need a common(open source) format for the avatar metadata (what meshes make it up, how/where are they attached to each other, what animations the avatar has, what textures they have, what shaders they use), a common format for the animations, the meshes, the textures. You also need a common name for avatar actions, walk, run, crawl, wave, etc. Of course everyone wanted their own proprietary format to be the standard. This doesn't even touch on more complex issues like currency, voice communications, video communications.
The problem is everyone has to find value outside of their proprietary systems, which 14 years ago was as much of a problem as it is now. Only outcome I can see is we will live with silo's until we get AI good enough to convert from one systems format to another's.
I'm sure you can share some interesting stories and/or arcs of what-was-envisioned vs. what-ended-up-happening.