A good skill for me is a skill which has mechanics and content that is well thought out and works well with the many other features available in RuneScape. I don't think it matters whether or not it makes money or uses money up because Jagex have already proven to us that a good skill can be either of the two. Runecrafting and Construction are two skills which are completely opposite in the sense that a player tends to make large amounts of money when RCing while if you decide to level construction, you're guaranteed to lose quite a bit of cash. However, despite their differences, both of them are much loved parts of the game because they offer so much to players that its insane to neglect leveling them. With RCing, there's not only the satisfaction of leveling such a respected skill to have a high level on, but it also does wonders to fatten your cash pile. With Construction, the benefits of a POH is well worth the money you spend to get to a level because of the ridiculous amounts of content that the houses brought along with them. So pretty much, the things that I personally think make a skill "good" are: the rewards, the mechanics and content offered and how well it fits in with already existing content in the game.