As most of you know, I'm gen for SG-1 (and SGA when I watch it) and prefer gen fic in the fandom. Gen doesn't mean the characters can't have strong or intense bonds. Gen isn't just emotionalless action. Gen isn't strictly canon. Gen explores the friendships and other kind of relationships between the characters in complex and rich ways.
However, that doesn't mean I don't like to explore ship or slash in fanfic. It gives me a challenge and I'm an idea person. If I get a workable idea, I'll explore it even if it's not normally something I would write. If I find a story that is interesting and convincing, I'll read it.
Some characters I can ship/slash more so than others. I've written a lot of ship with Sam, but it's not because she's the girl. It's more personality based. The same with characters like Daniel and even Cam or Teal'c.
The character I have the most trouble with is Jack. This is both for the show and in the fandom.
Emotionally, Jack is a very closed character. Sure, he jokes around and will do anything for people he allows into his inner circle. He's a great leader and isn't to be underestimated. But he isn't a very open guy. He has trouble vocalizing his feelings and would rather not go there. Now, in extreme cases that wall breaks down -- Jack holding Daniel in Need, Jack taking Kawalsky's hand in Enemy Within, Jack hugging/touching Teal'c in Lost City, Jack supporting Sam in Death Knell. Jack can express emotion, but it takes something to move him. While he's always been a hands on guy and likes tactile touch, that doesn't mean he's an extremely openly emotional guy.
This is why I have a hard time buying ship or slash with Jack. Jack would not be the one to take the first step when it comes to relationships, especially with people he works with and who he is friends with. There would have to be something intense to break that wall to get him to move. If not, it really falls on the other individual to push him in the right direction.
This is problematic in certain pairings. While Sam tries to explore this, since Jack doesn't budge it often makes her look bad. But she also is reserved and an emotional mess in many ways so I have difficulty seeing this pairing hooking up. In fanfic, it would take something serious for me to get them. Plus, I feel these two characters have a lot of respect for each other which further restrains Jack from action.
Jack and Janet I can't comment on because I've only ever really saw some spark early in the series. I haven't read Jack/Janet fanfic, but I would be curious on people's thoughts on this pairing.
Jack and other random women...I have an easier time with this, actually. Jack is a bit flirtatious and he obviously has no qualms becoming involved with women. It may be easier for me to see Jack/OC because it's not within the team and it's not really the focus of the show. On the show, these relationships are fleeting and don't usually take away from the team spirit. However, unless it was really really well done, I doubt I could read fanfic with Jack/OC as the main focus.
I also don't have much trouble with Jack/Sara. I'm not sure why. Again, it's non-team so I can get past the interteam friendship/coworker issues I have with Sam/Jack, Jack/Daniel, and Jack/Teal'c. (I don't see any basis for Jack/Jonas.) Plus, it's also a past relationship so the awkwardness of "falling in love" is over. I loved when past Jack/Sara was referenced on the show because it added another layer to Jack. Now, I do have a special interest in rekindled Jack/Sara but I'm picky. This pairing has a lot of baggage and it has to handled well.
Jack slash is even a harder sell. Let's say that Jack had some buried feelings for Daniel or Teal'c. For me, he's too much of a guy to even go there. That would be up to Daniel and Teal'c to open him, and I see both of them having similar denial or buried feelings issues. Plus, the military thing. This is probably because none of them have been portrayed as nothing but straight (as far as I can see.) Jack hasn't had enough interaction with Cam for me for me to comment, though I can't really see it. And I think there was too much indifference between Jack and Jonas.
So, for slash fic, it takes something special to convince me.
This isn't a slam on anyone's preferences. I'm just saying that it takes a lot of work and some expert handling for me to buy Jack breaking through that wall he has around himself. Some fic writers can do it and I applaud them. I've even tried myself - sometimes with success and sometimes not. And I don't just have interteam issues with Jack. I have that problem with everyone. I just can bypass the issue easier with other teammates.
Maybe if Jack was more open, I could ship him more easily. But then he wouldn't be Jack. I love him just the way he is :)