The first big shake up of the main SG-1 cast. For better or for worse (or for out of universe negotiating reasons), Daniel Jackson is no longer part of SG-1 (or, truly, this mortal plane), so instead we have Jonas: who’s basically a carbon copy of Daniel Jackson, but without all the history. I think he absolutely did a great job–and props to Corin Nemec for pulling that off–but I really wish that they could have done something a little more different since they were already shaking things up.
Great episodes: Descent (Jacob Carter, the team abandoned without support), Frozen (Ancients!), Prometheus/Memento (I love seeing the massive jumps in human tech), Unnatural Selection (more Replicators! bigger bads!), Sight Unseen (a surprisingly good Earth episode, one of the few cases where the Stargate secret really truly feels like it’s almost revealed), and *Full Circle (see below)
Less great: Redemption (I still don’t care for the Jaffa arcs and the slow build of Anubis' weapon is hard to build tension around), Nightwalkers (don’t care as much for Earth episodes), The Other Guys (it’s just too awkward for me)
Say what you will about Jonas, but Full Circle and the return of now-glowing Daniel Jackson is a pretty awesome episode. It’s the first time we really get a sense for how much bigger a problem Anubis is.