The campaign for DiabloIV was quite good and I was very excited when I reached the end game content. The paragon board gave the end game grind a purpose, and the helltides were enjoyable.
When they released the seasonal content I was beyond disappointed. Not only because they completely ruined the character I worked hard to power up, but also because the seasonal "realm" forces you to create a new character and start from scratch. That, coupled with already having played the campaign, makes the season UNFATHOMABLY BORING. You literally start from level 1 and have to slowly and tediously rework your way up to 50+ with absolutely NO NEW CONTENT other than the general introductory quest.
If they had simply made the season part of the Eternal Realm and allowed me to keep playing my existing character (regardless of the nerf), with helltides, and nightmare dungeons, and paragon board, AND the seasonal content.. then I'd still be playing.
As it stand, DiabloIV is not worth playing after the campaign. Wait for it to go on sale, play the campaign, then move on to a new game.