I chose to classify this as a pivotal episode because it introduced us to the Genii, who would continue to figure in Atlantis's fortunes throughout season one and two.
This was the first episode I happened to catch on TV, and I was initially drawn in by the fact that I recognized Colm Meaney from Star Trek: The Next Generation. What kept me watching, though, was the sequence where Sheppard and McKay find the bunker. Meaney was a bit too stodgy to make a convincing bad guy (Kolya, from "The Storm" and "The Eye," made for a much more menacing villain), but the snarky exchanges between Sheppard and McKay plus the cool reveal at the end of the episode made it entertaining. Shortly after that I decided to get caught up on the earlier episodes and I've been a faithful watcher since.