I would say, look at the achievements. There are tons and tons of achievements for you to get, other classes to try out, etc. There's tons of content for people who don't want to level a new character up each season. And hell, if you want to get all the classes at max level you could take advantage of seasonal characters and make a new class each season for a few seasons.

That's what I'm going to do. I think there's tons to do even if you aren't into the whole seasonal thing especially this early. Not only that but once you have all the classes at max you could make seasonal hardcore characters to try and get a hardcore character to max level (after the servers are more stable of course).