Addicting multiplayer.

Edit: still kind of fun

Edit Edit: hackers have taken over the game, valve customer support doesn't have the power to help customers - aka, if a blogpost is vague about something, expect your helpful Valve Support representative to emphasize how stupid you are in italics, don't expect a refund or any tweaking, Valve itself is an abyssal pit of suckage that would rather develop hardware for VR and storefront updates for Steam to generate revenue than hire a new team for work on their older titles to create updates that fix bugs, update anti-cheat and add additional major content for all titles.

I love this game, but Valve is a chaotic mess.