- an interconnected world of user-built locations, with near-seamless travel between them
- an environment in which every object is user-crafted and scripted, and can interact with each other and with users in a dynamic networked physics simulation
- a client which bundles the development environment, including 3D modeling and scripting tools, fluidly into the main game interface, such that objects can be created and edited on-the-fly and experimentation can happen live alongside other users in a social environment
- its own currency, convertible to real-world money and back via an actual exchange, and tools for conducting commerce both in-world and on the web via an Amazon-like marketplace
- infinitely-customizable avatars which allow for combining components from multiple sources and creators, spawning a micro-niche of virtual "fashion" trends, events, and blogs
What holds SL back is the accumulated cruft of ~1.5 decades of development mistakes [0], legacy technologies, and backwards-compatibility requirements with an ocean of content. But it's always been worlds more ambitious than modern claimants for the title of 3D social environment of the future, which seem content to be multi-user VR YouTube clients.
[0] I could go on and on about the mind-boggling pile of hacks that keeps SL ticking, but that'd probably drag this thread too far off topic :)