But the benefits in sandbox mmo is that a new player can easily get in on all the same action as veterans and they can also make a significant impact on the game. Unlike in a themepark MMOs a new player has to go through the same level up process as everyone else. A new player cannot participate in the same content as veterans. Not to mention, a player who plays with friends can easily find themselves behind in terms of content and gear. Once a player has fallen behind, he or she will not longer be able to do content with their friends.
In truth themepark MMOs are very demanding, you have to keep playing in order to keep up with the group. If you miss a few raids you can find yourself replaced by someone else. If you don't progress at at a expected average rate, you can find yourself behind with the general population that can cause you to lose out on many opportunities such as finding a good raiding guild.
In themepark MMOs its literally a race, a race to the server's first in max level, raids, and even crafting. Once you've fallen behind the pact, your impact has little value or usefulness. Nobody cares about the server's fourth, fifth, sixth and so on and progressive guilds prefer not to babysit new comers who are behind in terms of content and gear, in fact they ask you to catch up with their standards.
So in contrast, it's true that in a sandbox MMO you can lose your stuff but if you're a smart and careful player you can go through playing without losing any of your valuable goods. However, in themepark mmos you can easily fall behind not because you made a bad decision but because you may not have enough play time as those who are in your gaming network. In a sense, falling behind in a themepark mmo is actually worst than losing your items, because at least if you lose your items you can regain them back at your own pace. Where as, like I said before, miss a few raids who knows when you're back on the good side of the raid leader.