Based on the 1994 movie and focused on the use of a device that generates a method of interstellar travel, SG-1 has some of the feel of Star Trek. A group of adventurers travels to distant planets and meets various aliens. The basic structure allows nicely episodic breaks. We go to a planet, have a problem, solve the problem and face a new set of these next week. One of the features of the show is a set of long-lived aliens that pose as gods of the people they enslave using their superior technology. These are the villains of the show and we come to know them as characters, something facilitated by the fact that their names reflect Earth mythology. Ra is the villain in the original movie and other Egyptian gods show up in the TV series.
Stargate: Atlantis starts several years into the run of SG-1. An ongoing background has been built up as the team of adventurers has explored more of the galaxy. One part of this is that the group that made the stargate in the first place, known as "the ancients," came from Atlantis to Earth in the distant past. When it is finally found, a team of people are sent from Earth to investigate what might remain there. The set-up of the show presumes that the trip may be one-way, separating these men and women from all they know on Earth.
This is an interesting twist. They will have limited Earth supplies and no way to get resupplied. More importantly, they will not be able to draw on the vast knowledge and experience of the characters in the other show when things go wrong. They will have to be self-sufficient and mistakes they make could have severe consequences.
Before long, they discover the bad guys in this new galaxy. Known as "the wraith," they are a race of vampires that feed on then life force of humans and capable of hibernating for long periods of time. The flaw they have as villains, however, is that thye have no personality. That is, in all of the first season, we are not told the name of any individual wraith. they are faceless bad guys, easily interchangeable.
Further, during the season finale, they broke the cardinal rule of the show: They recieved assistance from Earth to save them from a big attack by the wraith. So now, the interesting twist no longer exists. If they don't have the resources to battle a threat, they simply dial up Earth and get what they need. I don't recall seeing a show jump the shark in their first show of their second season....