U5 also had a great gradual approach to discovery. First, you needed to survive in a hostile environment, gather a party and some basic resources. Then, you could gradually go around the mainland, visit cities and start gathering clues. You gained access to newer and newer modes of travel - mountaineering equipment, boats, ships, eventually flight - unlocking another level of the terrain, and with it new puzzles. You unsealed dungeons and discovered hidden areas, then found a whole underworld below the dungeons, with even more to come. Although Britannia was an open world, many of its secrets were hard to get (but also logical).
Ultima IV had a more obscure design, and it was extremely grindy; the later Ulltimas were good in their own way, but never had this satisfying exploration. In U5, buying items unlocked new subsystems, new ways of interaction and navigation getting you deeper into the game. This gradual sense of discovery is a key to its appeal.