I honestly don't think the people who have "ran out of stuff to do" have actually done everything (excluding the 1.0 players that came in with everything maxed, of course). It would be much better to say something along the lines of "I've done everything I find fun".

When you get to 50, technically you can level crafts, work on getting achievements, getting the best gear, leveling other battle classes, or getting pets, but not everyone finds each of these things fun. For instance, many people don't actually want to level other classes other than for cross class skills for their main class. They don't want to craft or gather because they don't find it at all fun and it's not really a beneficial thing to do unless you care about stockpiling money. Gear can only be progressed so far every week so that is typically out for those types after the first few days.

Others, like myself, find fun in getting the crafting classes to 50, and working on gathering things. Just personal preference really, and an error of players comparing content available in this game to content available in other games with several big patches and expansions out.