I understand that from a technical point of view it's an amazing tool that allows servers to handle things they otherwise couldn't.
However, it absolutely ruins the sense of immersion and community. There is nothing worse to me than playing on live and trying to meet up with someone on my server only to discover we're on different shards. We picked the same server and faction, that's all that should matter.
I understand that some people just don't see it that way and to them it's just a minor inconvenience, but a lot of us who want Classic want it for the community-driven nature of it, and it's not acceptable to sacrifice that for performance gains.
I would genuinely prefer to see smoke coming out of my PC when I run into Orgrimmar than experience Classic with sharding.