Got to agree on the vocabulary and just generally English skills have become better along the time I've played. I also had a chance to hone my German skills, which was very useful.
And something to do on those boring rainy - and sometimes on those joufyl and sunny - days. Lots of new friends and communication with people every day.
Runescape has created some fantastic friends for me as well as improving my social life which is restricted due to IRL circumstances. It constantly gives me something to strive for.
Runescape has expanded my vocabulary over 1,000+ words (I wrote them all done to remember them)(Does not include foreign words).
Runescape has caused me to be fascinated by culture, history, Arab culture, Arab history(Kharidian desert fascinated me as a kid), philosophy(Ethical debates/God debates/DEBATES in clan chats over the years), Anthropology in general.
Let's just say because of Runescape, I know WAY more than most at my age and sometimes I get to correct my history teacher's accuracy in lessons!