The price! It's so cheap that I can't really complain about anything at all, but still they could have made it even cheaper by not charging extra fees to use SSL or CDN features like Cloudflare which are very useful in many environments (even if you don’t need them).
They also offer great support through their forums - just make sure your questions aren't too specific as there will be people who know more than others here :) Support tickets rarely get answered within several hours unless we're talking of major issues with our VPSs/dedicated servers etc., otherwise expect delays up until days sometimes weeks before someone actually replies back asking us what was wrong again. We've been using virtualmins services since 2013 when my brother bought his first server from one of its packages offered then; he had no idea how much this service would prove helpful later.