The 48 page writ and other docs are here, I've only got time to look at part of it, but I will pull out the best claims and the most important claims for comments.
Standard disclaimer: not exactly my legal field, not my country.
I'm seeing contrary info on whether Canada/British Columbia applies the same higher bar that England apparently has for a public figure to establish defamation. Whether the plaintiff Andrew Weaver qualifies as a public figure under BC law is yet another question. Those two issues are likely the most important ones in the success of the case, and I have no idea how they'll play out. Let's ignore them!
Moving on, the good stuff begins on pages 10-12 of the pdf (pages 5-7 of the writ): two defamatory claims - that Weaver claimed the fossil fuel industry was behind two break-ins in his office, and that he doesn't understand "solar climate theory". The first one is a really good claim depending on evidence, while the second is the important claim.
Weaver denies he ever linked fossil industry to the breakins, so it's a simple factual dispute. The question is what does the evidence show - if there's a tape of the interview and he doesn't make the link, he's won the case (or this part, anyway). If there's no tape but his interviewer alleges he did make the connection, then it's a matter of who seems more credible.
Worth noting here that Canada has a "responsible communication" defense - even if the interviewer lied or negligently screwed up, no other co-defendant (besides that defendant's employer) is also liable unless they also practiced negligent journalism. Knowingly employing someone who's a bad journalist probably isn't responsible journalism though.
The interesting and important second claim major claim is that Weaver was defamed by the assertion that he doesn't understand "solar climate theory," something that a climatologist who studies climate change would be expected to understand if he were at a basic level of competence. This could go beyond fair comment/expression of opinion, something where the defendant better have some evidence on his side. The other interesting thing is I'll bet there's well-defined case law on to what extent you can allege a professional is incompetent, and when the false allegation gets specific enough to be defamatory. Obviously there's no precedent for climatologists, but that doesn't matter - similar cases made by doctors, could provide guidance.
On writ page 8, the claim that "models are all falling apart" when they're not, is also important. Mainly because it could get a judge to weigh in on computer models. The issue might get tossed as simple opinion, though.
The anonymous posters on page 11 and 12 may have defamed Weaver, especially the second one, but it's harder to see whether anyone would take them seriously, meaning their defamation wasn't capable of causing much reputational damage because they're obviously just two boneheads.
Page 14 "Doc Weaver was publicly blaming the oil industry" for the break-ins. Someone made a very big mistake by putting that word "publicly" in the article, because it doesn't matter then whether the interviewer claims Weaver blamed the oil industry. If they don't have a public expression somewhere to cite to, they're in trouble. Best claim I've seen so far. (UPDATE: thought about it some more - if the the interviewer will claim that Weaver made the connection speaking to her, then the defendants will say that counts as public. Kind of a thin reed.)
Kind of fun, on the same page, is the claim that McIntyre "broke Mr. Weaver's hockey stick". Weaver says (p. 15) that he had nothing to do with the Hockey Stick temperature record, and that puts the defendants in a bind: if a court says the hockey stick isn't broken, then they've defamed him, and even if the court somehow says it was broken, they've defamed him by blaming him for something he didn't screw up. Some wiggle room though with defendants claim that Weaver supports the Hockey Stick, and it's all just opinion, anyway.
That's enough for now, I'll have to come back later for more fun.
(One addition - the Deltoid link at the very top has a discussion of a case where Fox News was legally allowed to lie (allegedly). It's not a defamation case though, so it's not relevant to this discussion.)
UPDATE: second half here.