I am finding myself playing more and more on Old School rather than the shiny new RS. Looking at why I believe is that there are a number of things. First off it is more fun to play. The new game it is click handed or get for almost everything and in OSRS you have to work for almost everything. It is a bit of a pain doing all of the clicks, but, you are engaged in the game a lot more. You also have to think and plan what you are going to do for your skills and watch your player at all times instead of AFK training to 99.
The other thing is that OSRS does not have all of that extra CONTENT which from my point is just spam/crap and does not enhance gaming, it just allows people to AFK train and get a cheap way to 99 in a skill.
The last point is that OSRS is playable from any machine with a browser and Java. You don't need a fairly beefy machine with a great graphic card to play. I am playing OSRS on my basic netbook and enjoying playing a clean and simple game rather than trying to remember all of the things I have to do in the new game for combat or AFK training on the other skills.