RunUO's more customizable than Sphere, and more stable,
Look around in the script submissions for some examples of the sort of things that can be done with it. (That's thanks to using a full programming language for the scripts, as opposed to a smaller script language like the one Sphere used).
The scripting is harder here, than Sphere would be, but if you try something on your own first, and post the part that you're having issues with, (in the script support forum), people are usually more than willing to help out and give advice. However, *don't* ask for people to do things for you, practically everyone doesn't like that (or so that's what I see, and agree).
As for running a server, It's pretty hard to actually get a decent playerbase without putting in a lot of effort making your shard unique (i.e. custom made scripts, as opposed to downloading a bunch and tossing them on your server). By all means though, run your server and try out being an admin, see the nifty things they can do, just don't expect everyone to jump on your server just because you think it's cool
Also, as a note for the future if you start creating scripts, avoid copying scripts that someone else made, making a minor change, and then reposting it. Especially without giving prior credit. I personally couldn't care less about credit, but there's lots of people who'll flame you for that. And most people dislike scripts that are posted that are minor changes to the base scripts (that are included with RunUO). So avoid modifying a leather helmet to be named "God's Helmet", putting all the armor resist on it to 70%, and posting it
But, on the other hand, if you put a lot of work into something, and even though it's simple, it's unique (not renamed, or statboosted), by all means post it! I remember one submission that was a pocket watch, really simple, just click on it and it'd say the time, but it was unique! Plus it was from the previous Ultima's
There's my speech! Hope you find RunUO enjoyable!
Edit: Oh, and if you ever get an error from RunUO, a script, or anything, make sure to post it when you ask for help. It's difficult for people to help out without being able to see what the error is. And try not to post messages with the title of: "Help me!" or "Admin person, please read this", be informative about what the post is about