Ignore the haters. Every official announcement is being downvoted into oblivion at this point and most people only visit the subreddit to complain. The events have some minor improvements from last time.

Mayhem and Delirium are not too much different from the normal game, but the short duration makes them unattractive to many players. It's still fun if you have some new builds (that don't need gear) you want to play. Basically a full reset and some added challenge and bonus loot in every zone. Delirium was hated a lot last time because some early campaign zones had very high delirium amounts (giving 90%+ damage reduction) to bosses. This was supposedly changed now to a lower limit in early acts.

Delve is the most unique imo. It's just going straight down the mine, without any costs or limits. No trade or workbench is limiting builds a lot.

If it sounds fun to you, give it a try, but don't expect anything too special.