This is the first season I'm watching "live." I saw my first ever SGA episode in mid-May, and had watched all three seasons by the end of July. It's a different experience watching a show one episode a week, than watching it when you have a DVD boxed set and can watch multiple episodes in a row.
I've also been largely unspoilered. Although I initially tried to avoid spoilers, I did end up fairly spoilered for most of the big episodes of seasons 2 and 3, especially those episodes that spawned lots of fanfic. For season 4, I've read some pre-airing spoilers, mostly those on Joe Malozzi's blog, but haven't sought out anything else. The first two episodes I watched 11 days after they'd aired in America, but stayed entirely spoiler-free in those days. Episodes 3 and 4 I did, I admit, watch... er... well... though other means, the day after they'd aired in America. Remaining spoiler-free really enhances my enjoyment of an episode, since it feels exciting, fresh and edge-of-the-seat.
I'm also deliberately not visiting any forums or general discussion communities, so my reaction can stay fairly "pure." Of course I'm interested in other people's opinions, but I've had too many enthusiasms almost ruined for me by the vitriol of fandom. Opinions are great; "My way of seeing things is the only right way and you are wrong wrong wrong!" is not pleasant.
Anyway... I was worried in advance of season 4. I'd just spent a few months obsessing on old episodes, but it seemed that I was coming into the show just as it was changing massively. I've never watched SG-1, so had no emotional investment in Carter, and I was worried that she'd come in and push out my favourites. Fortunately for me, though, Weir has never been one of my favourites. I don't dislike her in any way, and I have a new sympathy for her after writing her in Entertaining Strangers, but I don't have any really strong positive feelings for her, either. I think she's been under-used - often reduced to standing around and waiting while Sheppard and his team for the exciting stuff. Of course, I probably have a very different perspective on things as a fan of three months than I would if I'd been invested in the show for years.
I mourn Carson terribly, but - for me - the show doesn't suffer without him. While I mourn the deaths of favourite characters, I like it when a show isn't afraid to make hard decisions. It decreases the tension, in my opinion, when you know that, however terrible things look, everyone will survive. That's one of the reasons why I love George RR Martin's novels so much - because no-one is safe, and anything can happen. The series feels that little bit more dangerous now, and I like that. (I do, however, wish they'd dealt more with the other characters' grief for Carson in the episodes that followed his death.)
As for season four... So far I really like it. Carter hasn't found a place in my heart yet, but it's neutrality I feel, not dislike, and I think they handled her introduction well. I find Keller likeable and think she's shaping up to be interesting (although I do still mourn Carson horribly). Most of all, though, I love the fact that this show seems to be turning into fanfic! (And I don't mean that in a bad way.) We're getting lovely moments of team interaction all over the place. Characters are actually talking to each other! Characters are evolving! Things that happen in one episode are mentioned in another! People have emotions! I see a very real difference between this season (at least as evidenced by the first four episodes) and the previous seasons in terms of character interaction moments, and I sincerely hope that this isn't a mere fluke, but will continue through the season. Everything I've read suggests that it's deliberate, and that it will.
Adrift: I loved it second time round. (First time round, I was too busy looking at it with external eyes, worrying about the changes in the new season.) I was truly on the edge of my seat.
Lifeline: Great moments, though I felt that bits of it were a bit rushed and needed more development. I didn't like the deus ex machina rescue at the end. (I never like it when the Daedalus turns up and rescues people in the nick of time, either.)
Reunion: I loved it! It was a Ronon episode, so I worried a bit that everyone else would be sidelined, but also included lovely character moments for everyone else, too, which is great. I think every episode focusing on one character should contain at least one scene of fan-pleasing stuff with every other character, just to keep everyone happy.
Doppelganger: Definitely favourite of the season, and one of my favourites of all time. Now to watch it again...