Personally, I don't see how "the DS's launch lineup" can be the biggest disappointment of 2004.
I mean, the system came out of freaking nowhere. Just... "boom"... we're here.
Sure, I think the DS's lineup is a little underwhelming right now (so is the PSP's, just to keep things in perspective), and it's not making me want to run out and buy a DS, but I just don't see how anyone can have been "disappointed" by what was, for all intents and purposes, a surprise (albeit, only a "mildly pleasant" one).
Also for the runner up, "RE4 and Viewtiful Joe no longer exclusive", ummm... Viewtiful Joe was never really "exclusive". It just "wasn't being made for the PS2", and then... it was.
Getting the shaft on RE4 by itself wins the "biggest disappointment" award in my book. Although after taking a step back, I'd say that the loss of Tales of Symphonia exclusivity (was that 2004?) would've been a bigger blow, if it wasn't for Mikami's repeated assurances that RE4 would never be on the PS2, and for Namco's general incompetence when it comes to releasing Tales games in America.
If you had a "RE4 and ToS no longer exclusive" or maybe a more generic "rats abandoning the supposedly-sinking ship" option, then I think it should've won hands-down.