There's an old saying, allegedly first spoken by a now forgotten programmer, "It was hard to program; it should be hard to play." Easy to see why the person didn't last long in the business
It's not just UO's depth that's a secret, the game itself is a secret to better than 99% of contemporary online gamers. I can say that with confidence because my line of work forces me to play every new online game, no matter how horrid it may be. The irony is that, although UO may have introduced online gaming to the mainstream, it was never a mainstream online game.
Were any new online game to debut with Ultima Online's player numbers AT THEIR PEAK it would be deemed a failure and would likely die young. In fairness, though, you cannot release any subscription based online game today. Heck, you can't even charge up front for a mobile game, but I digress.
Where the secret of UO's depth hits me hardest is on game developer forums. I've lost count of the times a clever young game designer announces their brilliant "new" innovation that has existed in UO since forever. And, so far, I've managed to avoid channeling the late Christopher Hitchens and instead have gently corrected these eager young guns while suggesting they try UO
As for the matter of the Ettins of New Haven, yes, I laughed the first time a new player - a genuine new player - proclaimed the moment their corpse was looted by an Ettin as their breaking point over the Help channel.
And, to be fair, this was always preceded by a long list of utterly bewildering events prior to the Ettin encounter. The Ettin was always the last straw.
Thus even if they didn't loot the young something else, such as killing your first mount by accident, getting lost for the zillionth time while trying to find the right NPC's head to drop gold on, getting killed by an accidentally targeted Wandering Healer,
or the countless other ways the new player experience not only fails to measure up to those in other games but fails to make any sense at all would provide that same breaking point as our greedy two headed friends.