An unique release with glorious quest design that got worse over the years.
The game has to this day a huge set of unique skills that no other MMO has ever come close to, and quests that can sometimes rival some actual singleplayer RPGs.
Many new upgrades are released each year, although I personally feel they've taken a cartoonish direction lately and that it's very far from the RS that I enjoyed some years ago.
I'd say that the game's most innovating aspect, although neither original nor appreciated by it's players, is the quest design. Each quest is completely unique and follows a short tale that builds up your character with proper rewards (obviously). Some quests follow story arcs, while some others are rather short.
There is also a huge element Runescape refined which is crafting. It's robust, easy to learn and very deep. It's intricately connected to the overall gaming experience, and I believe it fills a hole previously left open by 90% of other generic MMORPGs.
The graphics have always been criticized, and for good reason. They run in Java, and the game may feel clunky to play for some people. However, once you get used to them, the game's complexity and scope will keep you entertained for a long time.
I feel like the major flaws with Runescape nowadays is the direction the developer Jagex is going into. Switching to a cartoonish aesthetic over the classic one old players were used to is a rough change that might hide Runescape's true strengths.