The gear cap staying the same, not having to regrind your sets from scratch every expansion.
Eventually you build up a closet of sets....but let's not pretend we don't have to re-do it all every patch. I find myself grinding for sets every patch because all of a sudden my old build was nerfed into the ground and the meta moved to some crap that requires me to grind out Wayrest Sewers I for the first time in 5 years.
I love the consistent lore, the awesome voice acting, the range of activities, the actual mechanics of movement, the way Pvp is (when its not a bugged lagfest of doom), The beautiful art direction, the subtle ways each region is different from the other especially at the border areas when they meld, the glorious sunsets, the moons and starry skies, the delightful main NPCs...
I want to chillax and listen to an interesting story? I can do that. I want to take on a difficult challenge and try to emerge victorious? I can do that too. I want to just go kill some other players? ESO has that, although I wish it was better and only do it very rarely. I want to do something utterly mindless while I talk with a friend, just busywork? Well, ESO has that too with it's crafting and gathering systems, antiquities, etc. Do I want instead to engage in content that lets me be creative? I have that with the Housing and Fashion.
It's what has made it my most played game of all time. There's always something for me to do, no matter my mood or who I am playing the game with.
I love most things Elder Scrolls, but what has kept me coming back is the combat system. It really separates itself both from other ES games and other MMOs in general. Skill and effort should be rewarded. I think they have a struck a good balance.
Shame PVP performance is dumpster fire that makes 2020 look like hugs and rainbows.