MMORPG.com managed to corner Ultima Online lead engineer Derek Brinkmann in order to get his input on why sandbox gameplay is so appealing to gamers. A bit of what he had to say:
In Ultima Online, we provide open ended character development through the skills system. This gives our players more freedom to build their character's to suit their specific play style. In a level based game, character development often stops when a character reaches max level. In Ultima Online, players are constantly tweaking their build and adding new skills to their repertoire. We have objects in the game called soulstones that allow players to store off skills, leaving room in their template to train new skills.
When these concepts come together, they create a truly immersive experience. Our players feel they are living in a virtual world instead of just playing a game. While newer MMOs have become increasingly linear in both character development and overall game flow, sandbox games like Ultima Online remain open ended and provide players with an unparalleled sense of freedom. This is why we have players that have been paying for their subscription for over 12 years.