The problem migrates to AutoCAD being like C++ and having a long list of legacy objects that are never broken.
A competing format was headed by Bentley with OpenDWG but they abandoned it somewhat after RealDWG gave in to giving them the doormat for free.
My employer is a member of the Open Design Alliance [0] working on IntelliCAD. I work a layer up, building plugins for both AutoCAD and IntelliCAD. Free trials are not hard to find, so you can decide if it will work for you.
The UI is really nice. Version 3.0 whenever it comes out is going to be awesome. You may also try FreeCad for things this can not model, but that has its own problems - particularly UI.
The OSS world still needs a full-featured NURBS library that supports trimming. Solvespace does, but the performance suffers as complexity increases.