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I think that's the big difference between UO and the Level/Raid clones.
UO said "Here's the world from our single player franchise and here's a lot of little stuff you can do in it just like you could in the single players games and more!"
WoW etc. said "Here's a game inside our world, it's like a single player game, except where you will need anywhere from 15 to 40 people when we say you will."
In my opinion, that is the reason why other UO like MMO games didn't get huge success.
Do not mean offend, but young people rarely have patient to enjoy a sand box game, they need excitement in the first hour playing. Unfortunately, they are dominating the market in a certain level.
First off, I think the real difference is in how you play the two style of games at a more basic level.
In WoW and it's fellow EQ Clones, the player is directed through the game world. Go here for this, go there for that, follow the path the game had determined for you from zone to zone depending on your level. These games have to be built this way, because there's such a power gap between levels. As the player levels up, where he's been is no longer useful, and a new zone is needed. And with that, the game has to have a way to move you from old zone to new zone. So now they do it by quests, directing you where you need to go next. That's one of the ways WoW improved the themepark game design, by showing the player where the next content was in the chain.
In UO, you decided for yourself. You can go here, or there, and try to get what you decide you want, or build what you decide to build (character wise, house wise, collections wise...). That's why many gamers call it "freedom".
In current UO, they have walked right up to that line of separation. The idea of "a game for all players" is a bad choice. All games are "niche", including WoW. And they went full bore into their niche, polished it to a high shine, and that's why they have been so successful. Farmville, while not an MMORPG, shows that there is indeed another niche besides WoW. And that's a closer niche to what UO was made as. Look at the numbers. What's really the bigger niche market?
On top of that, if you look closely, watch closely, you'll see that there is a huge unrest among MMORPG players. These days, on neutral gaming sites, there's about an even split on what gamers say they want between Sandbox and Themepark. You can see many calls for "something different" and "Sandbox". You can also see that a great many WoW players are always trying out new games, but end up going back to WoW. They are looking for something different. They aren't finding it in anything close to a complete game, much less polished.
Attempts at new Sandbox style games are foolhardy at best. None of them have had the funds to make anything close to "complete". On top of that, they think that wide open PvP is the way to go, and it's most definitely not. Wide open PvP is for FPSers, not MMORPGs. It does not work on a large enough scale to provide the funds to add much content outside of that. Wide open PvP is "King of the Hill" gaming, and no one wants to be a "persistent loser". So they leave for a better experience for themselves, because they cannot compete against players who are driven on an hourly basis to PK everyone around them.
As far as Sandbox games being enjoyable, they can be just as enjoyable as any other game. They can offer all the same stuff as WoW, less the power gaps that lead to directed game play through zones. If you've played WoW, can you not imagine anything you do in WoW being in a Sandbox game? If you took UO, and made it new in a fully 3D game world, is there anything in WoW that couldn't be included? Even classes could be in a Sandbox game world, as long as they lose those extreme power gaps. Hell, we have "classes" in UO now, in a very loose (and very tailorable) form.
The key to a great Sandbox game is the exact same key to WoW. Polish, dedication to the game style, and lots and lots of content.
The thing is, Sandbox style is better in that freedom you get with it. And it's a natural for that "casual" experience that has a huge market out there. You could play Farmville in a Sandbox world. Or you can play "Raid content" in a Sandbox world. You just don't have to be directed through the world and jump through pre-designed hoops to do so, which is caused by said Power Gaps and the level/item grinds and zoned architecture that result from that.