Hello Everybody !

I'm not sure where the post belongs , but anyway.
When ( hopefully ) Dawnguard will come to PC , how dangerous a list of 40+ mods will be for the game to work properly ?

It shouldn't change too much. DLC Generally just adds new items/equipment. It's the patches that change formula's/data tables and screw with everything.

Some of the mods that change follower behaviour might have problems, in particular a certain well talked about vampire may clash if you have her as a follower.

Both of these answers are right. The only thing really affected is 1 follower. Unless your mod is in one of the cells that a new building is erected or a new cave is installed you should be just fine. If one of your mods conflicts you could always just remove it and wait for an updated version that fixes any of the incompatibilities such as cell occupation.

Well good to know !

Well, the Dawnguard CK we got pushed out was completely incompatible with our version of the game. So I'd be slightly cautious, it seems like a lot has changed behind the scenes.

It was a leaked, probably outdated version of the CK though, right?