[potatolicious]

Nothing has been as easily moddable as Half-Life, with the same install-base.

I suspect there's another problem with mods today: asset production. In Half-Life's day even I could hack out a semi-passable player model in a day or two. Nowadays, producing assets for games has been made dramatically more complex. In short, you really can't find a lot of people willing to produce that quality and complexity of art for free. The size of teams required to put anything together, as a result of this, has also put a lot of things beyond a mod team's modest reaches.

I think that's why a lot of the indie excitement is around casual and "low-fi" games like Minecraft. Assets have always been a major blocker, and when you embrace the fact that you're not AAA, you have a better experience with it.