Two-hour long events, quality roleplay that is leisurely and not rushed, individual participatory interactions (like asking for a volunteer to perform a task), huge decorated areas with immense detail and variety (that probably take a considerable amount of time to build), and focus on immersion. I admit I haven't visited every shard and there are many fine EMs at work in UO, but I do not believe anyone can find this level of quality, imagination, and dedication anywhere else.
A few pictures of my own: