The endgame content is the biggest issue imo. Compared to WoW where you can just hit level cap and jump straight into PVP battlegrounds/arenas or PVM dungeons and raids. On RS it takes 90+ skills and countless quests just to start doing bosses which often carry too much risk and hassle of planning to justify even doing them at all. Skilling and questing - yeah anyone can do them, but the real fun lies in the high end PvP/bossing and it just isn't up to par really.
I still enjoy playing from time to time obviously, but there are plenty of things to change such as adding random dungeon groups (not dungeoneering, it's a right pain in the ass with the whole floors thing and having to have the right level + find a group) as there are in WoW which give EVERY user that has reached some decent levels to experience end game content, rather than the vast minority that is actually able to currently.
PVP? Is a joke. Too many spec weapons, randomness and the pain of starting a pure just to have an advantage over others (who also have a pure making it mostly irrelevant anyway).