However, I want to be clear its not a marketing tactic in any way (re: hosh below, with an excellent comment!). It ends up being that implicitly but we waited and developed Vagrant 1.x for 5 years prior to calling it a 2.0 because we had a lot of goals we wanted to achieve: multi-provider, fantastic Windows support, stable installers, etc. We feel we've now achieved that in a very stable way, so its time to call it 2.0.
This breakpoint for us allows us to begin planning and executing on larger changes. Of course, we'll do all of this thoughtfully since Vagrant is definitely a tool you want to "just work" today and not think about breaking your envs. I admit this does happen from time to time though and I'm sorry about that, but we're getting better.
It's a less-important issue and just a convention, not a law, but normally v1.36.12 tells me "focused on stability and just working--boring but rock-solid", while 2.0.0 tells me "first release of great, new features--amazing but don't put too much weight on it yet". I wouldn't ordinarily think of 2.0.0 as the most-stable version of 1.x with 2.0.1 being the less-stable introduction of the great, new features.