Good version: the game has lots of content that remains meaningful at the level cap. Depending on your game, that could be a collection of dungeons, PvP, things to make or collect, areas to explore, or procedurally generated content that does not feel formulaic and repetitive.
Bad version: the game has lots of things to keep you busy and subscribed at the level cap. Depending on your game, that could be a variety of treadmills and grinds, or PvP and procedurally generated content that quickly becomes repetitive.
This is mostly an Explorer perspective, because I find it easier to think of what would make good post-cap Explorer content (new things to do) and bad post-cap Achiever content (do them each 5 times a day until you reach some new cap, to give us time to add more grinds to keep you busy until the next expansion pack). The distinction comes down to whether you would do X as an activity in and of itself, whether or not it produced some new numbers to add to your character sheet.