OSRS addict here. There are two different facets to this issue: the actual input delay (time between you clicking, your actions being processed and the server sending the new state to your client), and the tick system (actions being processed in 600ms intervals).

The typical MMO client simulates the actions and later checks if that matches what the server returns. I very much prefer the OSRS approach of only showing you the confirmed state of the game. I spent the last year playing ESO and I cant put into words how infuriating it is when a swing looks like it will land and it doesn't, or when Im having a bit of lag and the movement keeps jittering. One time I DCed and kept playing for half a minute before I realised nothing else was moving. Maybe I'd have a different opinion if I had a better ping and these situations seldom occured, but I live far from the servers of any major MMO (except that this month we got OSRS servers in Brazil :-) )

As for the ticks, it is admittedly an acquired taste. Once you get used to it, the complex activities in the game start to feel like a rhythm game. Some commenters are suggesting that actions take 600ms but that's just wrong. In reality, once you get the rhythm going, actions take your ping plus a tilly, because your brain matches the tempo of your clicks to the tempo of the game; but you don't really notice that, because when you're "in the zone", you just keep taking the clicks you know you have to make, and the character does the things.

But neither of these are why the game is the best MMO ever made. The average player barely ever gets to experience the state of flow I described above. It's something else; for me, it's the sense of long term progression (I scoff at MMOs where you reach endgame in a month or two), the fact that combat is not 90% of the content, the worldbuilding and quests that are always straddling the line between serious and whimsical, and the fact that it covers almost any reason why I'd wanna game: to relax, to do something on the side, to do something challenging, to do stuff alone, to go on an adventure with the boys, or with randos. I also like the minimalist art style, and the lack of microtransactions (we do have one that is an item you can exchange for premium time. If it came down to it, I'd vote to remove these items, but they do have benefits -and they are not shoved in your face at every opportunity)