Welcome back! Barter Town is making a come back, be sure to stop in and say hello.
Mariah's link pretty much says it all, but here's a few highlights and a Siege perspective:
In the past 4 years:
ALL accounts (including yours if you did not have it before, and accounts created now) include the Stygian Abyss expansion.
Time of Legends came out in late 2015. New lands, new quests, new bosses. ''The Roof'' is currently where the action is for many Siege players. Depending on when you last paid your account in 2015, you might have ToL as there was a promotion that granted it to every active account at the time. An important game mechanic that requires ToL is Masteries.
https://uo.com/wiki/ultima-online-wiki/skills/skill-masteries/Endless Journey is an unlimited time free trial that replaced the 2 week trial. There are a lot of restrictions, but players can enjoy sample-sized portions of everything UO has to offer, for as long as they want, without subscribing. This has impacted the ''one character'' rule on Siege. Trial accounts that were never paid were not converted to EJ, but you can fire up any other dormant accounts. If you do, tho, read up first. You won't be able to re-mount a giant beetle, for starters, so don't dismount if you decide to check it out.
https://uo.com/endless-journey/There was a pretty big overhaul to the BOD system. New BOD skills, new rewards. BOD books can be used while locked down, instead of having to make room in our backpack.
https://uo.com/wiki/ultima-online-wiki/gameplay/crafting/bulk-orders/Taming got revamped and is pretty much amazing imo because now there is a much larger variety of powerful pets instead of just Greater Dragons everywhere. You can find people taming pets in a lot more places, as well as training them. Pets can have power scrolls applied to them, which has created some interesting value fluctuation for the scrolls, and more people doing the champion spawns to get the scrolls. There is a non-transferable token from the store that increases max stable count (per character).
https://uo.com/wiki/ultima-online-wiki/skills/animal-taming/A new champion spawn was added, in Khaldun Dungeon. Not many people on Siege run it because it is very challenging for the same PS as a rat spawn, but the boss always drops a few Costumes. A store item can permanently bond a costume to a totem, which not only frees up your hand slot but blesses the costume on Siege. These are especially fun when worn by gargoyles. Expect to see players looking a bit different on occasion.
Each publish for the past year or so has had a housing contest with it. There are a lot more styles of Castles and Keeps to choose from, so the landscape of Siege looks a bit different.
There are a lot of other changes, but this post probably already exceeds the request of ''brief overview''. Our devs have been busy!