Runescape, like many other MMORPG's, is addicting. Not everyone can get addicted to it, but most people will at least have the potential to do so. Here's why:
1) Runescape uses a reward system that gives a lot of rewards right away, but over time the rewards are spaced farther and farther apart. Getting to level 3 in a skill may take only a minute, but getting to level 30 may take up to an hour, and getting to level 99 will take hundreds, even thousands of hours. This strategy can keep a player hooked on the game, because each level is only slightly longer than the last.
2) Runescape provides a player with a virtual world and an anonymous identity. If a person finds their life to be difficult, here the Runescape virtual reality is a place for them to escape into. They can make new friends, and because of how anonymous they are, they don't have to worry about physical appearance or social status. They may also experiment with a new persona. A shy person can become outgoing, and a passive child can become aggressive.
3) High Scores. Everyone wants to make it onto the high scores, and so they may play more and more to try to get onto the high scores list, often trying to reach 99 for the skillcape. Competing with friends can also be addicting, as you may want to be ahead of all your friends in at least one particular skill.
4) Leveling particular skills help level other skills. For example, with a high mining level, a player can then mine all of their ores, which makes smithing easier. Also, leveling in certain skills means you can unlock new quests and new areas to explore, making the game more and more interesting as there are new things to do and see.
5) Unlike other MMORPG's, such as World of Warcraft, Runescape is very cheap, and can even be played free. Without paying anything, you can become addicted to runescape and then later get bored and buy a membership, when the game gets even more addicting as there are new skills and items and areas to explore.
They're completely different types of games. WoW is a massively multiplayer online role-playing game, and Halo is a first person shooter. It's not really fair to compare them as they're both worthy games, but in different categories. The gameplay is too different to make an unbiased decision. Maybe you could get a month of WoW and rent Halo 2 and see which one you like better. There's no fair way to answer this question because there will always be fanboys of either game, ready to defend the slightest criticism. Likewise, there will always be haters of either game who will flame it regardless. A lot has to do with if you prefer a medieval setting or a futuristic battlefield.