Jainia betraying Kul Tiras hadn't been mentioned in WoW, it's also relatively obscure since it happened in a patch to the expansion of Warcraft 3. So it's significant to look back and go "this is a really important event and the Alliance campaign is all about it".
The new information is that Jaina regrets what happened so much and is now ready to return to Kul Tiras and make amends, we also didn't know that she is such a legendary figure in Kul Tiras to the point that they have a folk song about how confused and hurt they are by her actions. Also giant flying boat.
The Azshara one has literally NO new background on Azshara, what did you think was new? It's 100% background so there's no hints at what is to come (unlike the Jaina and Sylvanas ones), which is my main objection. But yeah, if you've been reading the in game lore books and quest text then you would know this story - before Warlords everyone was guessing this would be the expansion story since it's the only bit of lore left.
We knew that the Old Gods saved the Naga from the sundering because Azshara made a deal with them. We knew that N'Zoth is the last Old God left who has an evil underwater city (Ny'alotha), which you could guess was going to look like Ahn'Qiraj / Azoj'Nerub. I suppose we didn't know that Azshara and N'Zoth have a more equal relationship than you'd think.