Just like "Quarantine," I thought "Trio" would be a pointless episode at the beginning, but I ended up enjoying both episodes a lot.
McKay was hilarious when he kept trying to convince Carter to flash her you-know-whats to the boys to get them to find help. "No, Rodney! I'm not going to!"
Like some on the forum, I too was slightly disappointed that Carter found the rope. Or at least by the fact that she couldn't rummage through the locker for a couple more minutes before finding the rope. (You'll have to watch the episode to understand. And it helps if you're a guy. ;) )
I wasn't aware that the Keller and Ronon relationship from "Quarantine" didn't work out. Maybe they realized that the only reason they almost kissed was because of the shared crisis situation they went through, and that they don't really have anything in common.
McKay has shown a lot of character development since the beginning of the series. He never went on missions before. He didn't really know how to fire a gun or chase down bad guys in the field. His attempts at wooing Carter on the SG-1 series were pathetic so his relationship with Katie was a marked development in his social skills. He actually admitted that he could be arrogant, something I doubt he would have done two years ago. And he has even shown something like compassion on a couple occasions, such as when John lost a relative. (Of course, Rodney still doesn't get along with children very well, as we saw in this episode and in "Harmony." I wouldn't mind if he didn't develop in this area because his interactions with children are pretty funny. "Listen you little brats! I'm in control here!")
I think Keller was just asking McKay out to the bar as a friend. I don't think there was anything romantic in the request. They had just been through a tough situation together and Rodney did save her life. It's natural to want to hang out together afterwards as friends. I don't think Rodney is her type, and she understands that too. She won't let that prevent her from developing a non-romantic relationship with him though.
While there was nothing eye-opening about the story, the humorous moments helped to carry the episode. If I had to point out a flaw, it was the frequent falls of Carter. She fell from a height of over 25 feet at least twice without suffering any serious injuries. It was only on the third fall that she finally broke something. Apart from that, I found the episode entertaining and funny, and I learned more about the characters too. Not a bad way to spend an hour for a sci-fi viewer.