[quote name='dallow']According to who?
OXM reported that it's just some extra missions that aren't a part of the main story adding about 10 hours of gameplay.[/QUOTE]
Okay, maybe "dozens" was an exaggeration of their claims, but they insist that it's pretty

ing huge. And yeah, no duh they aren't part of the "main story" of the regular game; they're supposedly entirely new episodic story branches (apparently in new locales), not just extra missions for the main story.
Remember, Microsoft paid 50 mil (which is several times the budget of your average top-tier current-gen game) to keep these things exclusive. They must be pretty damned substantial to justify such an investment
Additional info:
[quote name='xbox.com']GTA IV will take the form of two distinct episodes, both downloadable from Xbox Live and each a minimum of 10 hours long.[/quote]
http://www.xbox.com/en-gb/community/news/x06/gta4
[quote name='uk.gamespot.com']"We're not talking about an extra car or character here," Moore said when describing the "epic" episodic packs. He then joked about Rockstar keeping even him in the dark about the content packs, but emphasized that the packs would be lengthy.
A press release issued during the conference said only that the packs would add "hours of entirely new gameplay" to GTA4.[/quote]
http://uk.gamespot.com/news/6158823.html?sid=6158823
GTAIV DLC
* When MS says exclusive, they mean that no other system can have DLC content for GTAIV
* It will be bigger than a lot of people think. GTA3 Vice City and San Andreas were basically expansions of GTA3. Think of the DLC that way. We are talking major expansion for GTAIV
http://www.thebitbag.com/2008/02/21/microsoft-blogger-breakfast-bullets/
[quote name='CVG Magazine']"Of course, to call games as vastly ambitious as Vice City or San Andreas mere 'expansion packs' seems childish, but nevertheless, the downloadable content (DLC) coming for the Xbox 360 version of IV has repositioned those games in just this way. DLC so far has meant the odd new car, jumper of bit of horse armour, but GTA IV is set to completely redefine the idea with expansions that are to GTA IV what Vice City or San Andreas were GTA III. Yes, Rockstar is clearly hinting at new downloadable cities; and the chances of them being London, Vice City or SA again are slim to none. So that's new as in brand new. GTA IV's Liberty City is the beginning. Think about that and be excited."[/quote]
http://www.playthree.net/2008/04/cvg-gta-iv-xbox-dlc-will-be-bigger-than.html