Except this doesn't give you new factions, weapons or mechanics with the "addon".
This is not completly true.
The point is what you want to call new in that comparsion.
I know all of the W40k factions before they where in the PC-games. I just got over to the next phantasyshop and read the codex for the factiong. People who are actually playing the game where even less suprised.
But that isn't the importend point to me.
Each pack should contain the storyline of one of the factions, a unique playingstyle, terrans AKA Raynor have a more "mercenary" playstyle. Protoss shall become more concerned with diplomatic decicions like what of the many different Protoss tribes you want to assist or ally with and Zerg isn't written in stone yet..at least I don't know anything about it.
So here is a hugh differance between the games on how you play them in singleplayer. Even more the between spacemarines and Tau when you want to take the SC2 - DAW comparsion.
You will also have some unique weapons for each faction in the singleplayer mode. I heard, for example, that Raynor might have to buy some old units like Goliath since he can't affort the newer Viking.
I think, if they will do as they promised its more then just an addon. Its nearly a complete new game sharing the same engine and backgroundstory.
As much as I dislike the splitting and also had the "cheap rip off" thought when I first heard about it I think one should just keep an eye on it.
About Blizzards storys. Well the ingame parts are mostly...simple. On the other hand they had these nice manual and I found it to be a good read. Sure it wasn't the holybook of whatever but good.
It gave you quite a lot of information about the different species, their past and development and a view into the different factions inside of each species.
The game itself was just the last part of a long ongoing conflict. At least that was my impression. Also they have allready said that the limits they had back then prevented them from showing much of the storyline they had planned.
As for WarCraft...I was never that much interested in it. Played it on LAN but it didn't catch to much of my attention. I just prefer Scifi enviourments.