I played Everquest a lot back in the day. Not hardcore amounts of hours but maybe 2 hours in an evening. Like most MMOs the game starts off with easy quests and lots of exploring but as it progressed levelling up really began to drag. By level 20 it might take 2 weeks to level up. I found myself camping more often. I found myself repeating the same damned action over and over - Meditate, Buff, Kill, Retreat to safe area, Meditate, Buff, Kill etc. For variety I might stand in the tunnel attempting to auction jewellery. A good session might see the blue xp bar advance a few pixels. A bad session end with a fraught corpse dragging expedition (or two) and less xp. Hauling ass over the map might take an hour. Boats might take 20 minutes to appear. Spawns might happen once a day and of course were camped out.
And I put up with this bullshit because the transition from fun to grind was so gradual I did not see it happen. So there I was paying subscribing to a game I didn't enjoy. Fortunately for me Verant intervened with their own ineptitude. The Shadows of Luclin expansion was bugged to high fuck which meant the server crashed, the client crashed, the content was bugged out and this went on for weeks. It gave me the time to realise I wasn't enjoying this.
So I let my subscription expire and I quit. It was a wrench to abandon the "investment" I made in the character but it just wasn't fun any more. On the plus side, it trained me to recognize grind and skinner box style gameplay that virtually all MMOs since have used to string people along - long travel distances, infrequent spawns, equipment that degrades, time sinks everywhere. I played other MMOs - Dark Age of Camelot, City of Heroes, Lord of the Rings Online, Star Wars Galaxies, A Tale in the Desert and they all suffered from them. Ultimately I quit them all because they were the same damned thing - sucking $15 out of you each month in return for anti-fun.
That said, with the change to free-to-play model has made some MMOs fun again. Lord of the Rings Online for example has been aggressively cutting the grind all over the place - adding fast travel, instant looting, less maintenance, out of combat healing, NPC radar etc. Presumably in the FTP model it pays to get people to progress more quickly rather than have them fuck around looting corpses or recoup lost xp. It's also a very beautiful game with the lore to support it. I've been playing LOTRO for 18 months in the FTP model and must have bought about $50 of points on it, most of that still remaining to be spent. If I don't feel like playing I'm not losing out by not playing so it suits me a lot better. I can play it for 30 minutes during a bout of boredom and feel like I'm getting something from it.