Joe Flanigan was great as John Sheppard in Stargate At, but I started to loss interest in it when it became too much about him. The show writers let his character take over the show is season 2 and after.
This is why Torri Higginson left the show at the end of season 3 and why other characters did not improve (Dr. Carson Beckett) in the series. This is why the show did not last as long as SG1, it became too much about John Sheppard and fighting the Wraith.
Many fans would agree as the show started to loss rating after Torri Higginson departed.
By season 4 not even Amanda Tapping and Richard Picardo jointing the show could save it and why they made a deal with Sifi channel to bring the show to an end, when Sifi was just going to cancel it after season 4.
Another show that did the same thing was Star Trek Voyager once Jeri Ryan jointed as Seven of Nine (a Borg drone) the show quickly became too much about her in seasons 5 and 6 almost all of the episodes was about her and the Borg.