I think the addicting part of the TES series is the sheer amount of things to do. You get overwhelmed really quick and it seems really impressive, like "wow, look how much stuff there is in this game, I better get to it!" and checking quests off your list is fun at first.
Then eventually, you get so powerful none of the quests are a challenge, and you come to realize that virtually all of the quests are the same, and the fun level immediately crashes down to zero.
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