They are all similar in the nature of what you have to do and how they play. The main aim is usually to spend a long time doing runs to and from the same placed, killing the same creatures over and over again to collect specific items they drop or money. The aim of which is to buy or craft better weapons, ships, armor etc... for your character. To complete main quests to gain specific items or access to further regions. To level up and become more powerful.
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The problem with them all is that you do need a lot of spare time on your hands to get the most from them. You need to keep working at the lower level areas, collecting items, gaining gold, and moving up through the levels to become powerful enough to take on the next areas more powerful enemies. This is much the same as any traditional RPG, but with online games it requires a lot more time and effort to get anywhere.