A very niche game - its one of those you either love it or hate it...nothing much in between. I personally like it.
A lot of RPG games follow a linear storyline, maybe adding some twists and turns as the story progress, giving multiple endings and other re-playable value...or, the game is simply magnificent (like FF series). RS, however, takes itself away from these categories to make a style of RPG all by itself - it is non-linear and the player is free to venture to wherever he/she wants to go or do. I understand how many will be put off by this gaming style.
Regarding the quests, indeed some are purely single quests to improve your party, but many of the quests in the game are linked e.g. opening up a new area to explore or a character to recruit, gives you a new rumor about certain characters and events, giving you artifacts that help in the final battle, etc.
In a sense, one can almost think of it like GTA - at first, there is a lot of freedom of what you do in the beginning, then as you progress in the game, all the quests and information start to lead and aid you towards one final goal - for RS, the goal is to defeat Saruin. Simply put, the goal of the game is to defeat Saruin, how you go about it is up to you.