Detecting methane per se isn't what's interesting about this. You don't need life to produce methane; there's plenty of it in the outer solar system, Titan for example is covered in the stuff, and it's been detected on Mars before. But it's a small enough molecule that Mars daylight temperatures can give it enough velocity to escape from the planet's gravity well, which means that if you detect nontrivial quantities in Mars' atmosphere it's probably being actively replenished somehow. Fluctuating levels of methane, which is what's actually new about the Curiosity measurements, are strong evidence for active replenishment.

This doesn't necessarily mean life -- there are other possibilities involving deep geology of various kinds -- but life is one of the candidate explanations. Though if it is life, it's probably simple, boring microbial life similar to what appeared quite early in Earth's history, so I think artemium is overselling the Great Filter implications on a couple of levels.