I've been using an OpenSim-legal viewer in Second Life for several weeks, and I don't have any problem with the missing features. It's essentially Mesh uploads and some aspects of building for the Pathfinding system. If you don't build, it doesn't matter.
Linden Labs are doing a lot of work on switching from UDP to HTTP, and on using more different support servers. and that all could affect viewer compatibility. The new Server Side Appearance has been delayed a lot, but it will be another barrier against OpenSim, and another obstacle to content theft. They don't mention the content theft implications.