Stargate Atlantis follows a mixed group of scientists and explorers from a dozen countries to the lost city of Atlantis. It all starts with the discovery of an ancient base in the Arctic (could be the Antarctic, don't remember) that leads to the discovery of the lost city of the Ancients, Atlantis. Just one problem, it's in the Pegasus galaxy, which is where the ancients were and the power available to get there is enough to make just a one way trip. With that in mind, the trip is made and the adventure begins.
In this new galaxy the team comes across all sorts of interesting foes such as the Wraith who are the Pegasus galaxy's version of vampires, responsible for the extinction of the Ancients, and the primary antagonists of the Atlantis expedition. Many of the episodes deal either directly or indirectly with them, and prove to be a quite resourceful, and exciting to square of against.
Replicators (not quite the ones found in SGC) where the other big threat to the Atlantis team. These replicators however were actually created by the Ancients to fight the Wraith but then abandoned because of their imperfections. This caused them to hate the Ancients, and the Atlantis team. Like the Wraith, they were formidable foes and interesting to watch.
Lesser foes included a human civilization called the Genii. Probably the most advanced of the human civilizations in the Pegasus galaxy that started out as allies but a few plot-required incidents and their taste for Atlantian technology put them at odds with the Atlantis team. A character called Michael who proved to be quite a thorn for Atlantians. Him and his minions came very close to capturing Atlantis but at the last moment were thwarted.
Besides dealing with the bad guys, the show also had a lot of episodes that dealt with development of the various characters in the series and how they dealt with love, friendship, death, and just the internal struggle for power to lead Atlantis, which turned out to be quite a contested job.
I personally was never a big fan of the Stargate franchise until I started watching Stargate Atlantis. I found the few episodes of SGC that I watched a bit too cheeky for my taste. Almost everything was a joke to O'Neil that for me just ruined the entire experience of watching an episode. Atlantis on the other hand had a decent balance of humor and seriousness. I found McKay's character very annoying and the writers could have definitely reduced his constant need to talk, especially the over the top yelling (which he did often) but other then that the characters were pretty relatable.
My favorite was Ronon. His portrayal of the muscle of the team was very authentic. The part of Dr. Weir was also very well played and I was sad to see that Character leave in season 3. Sheppard was also pretty good, although many times he seems to channel O'Neil and that at times took away the seriousness of a particular scene.
The season 5 ending (and the series finale) could have been used to create some incredible stories for 6th season but alas the show had to end at some point. Overall I would definitely recommend this show to both fans of SCIFI and non SCIFI. It may not get your heart pounding or make you jump off your seat (everybody's different so I could be wrong) but it wont bore you either.