QUOTE
If it weren't for all the companies that make crappy games, it would be obvious that WoW -- a game designed for commercial operation from the start by one of the most famous game companies in the world, costing about $50 in the store plus an additional $13 (or so) a month -- would "pwn rs in teh buttzor wit itz huge peenorz" given that RS was originally designed as one of Andrew's "on-the-side" hobbies, and even you only have to pay $5 a month to Jagex -- one of the lesser-known gaming companies with no reputation outside of RS -- and the members' version is optional.
For about $22 per month (Sony's 'Station Access' plan) you can get Everquest, Everquest II, Planetside, and Star Wars Galaxies, which all also 'pwn' Runescape.
All of those games and WoW have a huge backing. Also games like those are already above a browser-based Java game in terms of capabilities.
The thing Runescape has is accessibility and simplicity (not in a bad way). Can multitask and do whatever while also being on RS. Also easier to bring friends to RS since it has a free version, making the social aspect a little stronger. Can also play it from basically any computer with internet access. Not so easy with the more massive games. To some this may be a consideration.
I'm personally an EQ2/SWG person, but every game mentioned in this thread has its strengths and weaknesses.