Why I like Stargate Atlantis
I also enjoy that, as a spinoff, SG-1 and Atlantis are much more closely related than many spinoffs or franchise shows. To use an example I and many people would be familiar with, let's consider Star Trek: The Next Generation and Star Trek: Deep Space Nine. While there are some connections there (particularly the characters of O'Brien and Worf), it's mostly a case of "Hey, let's look at this OTHER interesting group of Starfleet people." Here's how TNG and DS9 might be if they were related in a similar way as SG-1 and Atlantis:
The Enterprise-D is one of very few Federation starships, particularly in the region of space it works in. Over the course of a season or two they become involved with the situation on Bajor, help the occupation come to an end, and discover the wormhole to the Gamma Quadrant. Having done that, they help set up Federation control of DS9 as the new spinoff begins. From there on, being the two premiere Starfleet presences in that area of space, the activities of each show are occasionally referenced on the other, and guest appearances from one show to the other are not uncommon.