Here's my perspective --
I've played WoW pretty hardcore for a little bit less than two years before I quit. For me, I can actually say that the experience was beneficial to some extent. This was around 4,5 years ago, before I even knew HN existed.
Before I started playing the game, I heard some of my friends talk about raiding. For people that are not familiar with the concept, once you reach the maximum level in the game you join a guild. Once you're in that guild, you can go into dungeons with people from your guild and slay bosses. These bosses drop items that in turn allow you to upgrade the gear of your character. The cool thing about these bosses is that some of them actually quite challenging to beat. Once every couple of months, the developers of the game add a new dungeon that you can clear with your guild. They were also talking about these high end guilds that apparently consisted of insanely good players that would clear these dungeons before the masses did.
To give myself a challenge I decided to play the game but with a goal in mind, join one of these guild. Once I managed this I would quit. I began as a noob. I levelled up a character and joined a guild. Once I outgrew this guild I joined a better one.
I played for around a year in this specific guild. While playing here I actually met two people that I would call friends. Their background is so different from mine that the chance is so slim that I could have met them in real life. We've met up several times (in real life) and if I needed their help they'd be there for me. In this guild I was also in charge of leading the group of players through the dungeons. You're in charge of communicating how to do certain things and during the fights you give guidelines if something goes wrong. I raided 4-5 days per week from 19:00 - 23:00ish in this guild.
I then managed to join the guild that was N°1 at that time, together with one of my friends from my previous guild. In this guild, it was all about achieving the world first kill of a boss. It's great when you arrive at a boss and you have no idea as what to expect and how to kill it. It can be a pretty hard puzzle sometimes. If you're not there as one of the first you can read up on proven ways to handle the fight, which is less challenging. Also, contrary to popular belief, these guild usually play less then the other guilds. They go all out when a new dungeon is released (1-2 weeks) and then they play one 5 hour day a week for 4-5 hours a day and they wait for the next one. The funny thing is, the majority of the people that were playing here were also working as lawyers, programmers or were entrepreneurs. I spent a couple of months with the guild and once we cleared the last dungeon and had to wait for the next one, I quit. After that, I also quit the game.
Many people told me I was addicted to it, but considering it was rather easy for me to quit I'd say I wasn't. I was working towards a goal.
So what have I learned? I personally see life as a game. You win some, you lose some. Regardless of what you want to learn or achieve, you can. Also, communication is important in whatever you do, especially when you're in a leadership position. Oh, and I had a great time playing it. :)