I have GM experience working on a private MOP server. I was one of the GMs on Karabor, which was an affiliate of Molten at the time, if anyone remembers. I watched the progress of our server, and most issues stemmed from CLASSIC CONTENT needing major overhauls. Our team was small compared to Molten, but even with the assistance that Molten was providing, it wasn't enough man power to reprogram the game from scratch.
I personally think it failed due to the constant NEED to go backwards, instead of programming the initial expansion. We also moved to warlords of draenor quite early, and that left alot of "fixed" issues to be broken again. During that time, we also lost partnership with Molten, and everything went into a downward spiral. With that said, I still loved our team and loved my job there.
What I'm trying to say is, the drastic changes in gameplay made it difficult to code. Instancing was probably the most broken aspect from what I remember. Asking for a server beyond WOTLK, which has been worked on for YEARS, is unfortunately asking for too much. Just be happy with the experience you have here, because it is near perfect in comparison to what you could have elsewhere. :D