Runescape was crazy for the time (pre-WoW). A full-featured MMO you could play on a wide variety of computers without downloading or installing anything. All you needed was Java.
All you needed was to download a new monthly version of 50MB of JRE and install it on your virus-prone Windows XP machine and open up about 7 new holes in your system with each patch.
You probably had the JRE installed already, unless you were particularly security-conscious. Even many who were security conscious had it installed. Runescape was also compatible with the MSJVM, though the window of time where that mattered was short.
