Threat management isn't fun and in fact it's extremely frustrating to deal with. It made doing things like carrying your friend's tank alt through random dungeons as an end-game raider (and before LFD most runs in guilds were carries, in my experiences) virtually impossible to pull off without deaths.
It also created a massive disparity in what tanks were largely loved in LFD groups and which ones were loathed simply due to how reliable their threat generation was. I know I'd silently groan to myself every time I saw a bear tank anywhere; not because they were bad tanks, but because their threat generation was absolutely awful compared to, say, a prot paladin.
In general, it was an outdated and frustrating mechanic and I'm glad to see it go as someone who enjoys both tanking and dpsing. I'm all for the important, less popular roles being made easier to pick up and I think most players should be, too. An increase in tanks makes the game more enjoyable for everyone due to faster queue times and more availability for guilds.