In DoW1, you pretty much built units, threw them at the enemy, and anyone left alive kept advancing.  You didn't need to worry about defense since you had turrets, mines, and turrets on your resource generators.  You could also reinforce anywhere so it was easy to hold territory and maintain an advance.

In DoW2 you are forced to be all tactical and sneaky-like, and there are almost no turrets and reinforcing requires the use of a squishy transport, teleport relay, or your single HQ structure.  The only way to hold territory in DoW2 is to have troops there, and troops cost upkeep so its hard to amass a large army.

In terms of racial differences between DoW1 and DoW2, they're big.  Everything got reduced in size.  Fewer squads, and fewer members per squad.  This is great for Eldar, Spess Mehrines, and Chaos, however Tyranids and Orks have freaking huge armies with near-limitless numbers (there's even a special rule for Nids in tabletop called Without Number that gives reinforcements) in fluff, so it makes far, far less sense for them to have squads of only a few units.

DoW2 should not be considered a sequel to DoW1, simply because it plays so differently.  It's not like C&C4, as that game sucks (it copied DoW2 and very poorly) while DoW2 is good if you replace the "Dawn of War" in the title with something else.

If you want epic large-scale battles like the grimdarkness of the future is popularly known for, buy Dark Crusade.  If you want a game that plays a lot like Company of Heroes: 40K Edition (just without the awesome variety of units CoH has) then get DoW2.

If you do get Dark Crusade, I command you to grab the 3.5 version of the Firestorm over Kronus mod.  It will rock your socks off.  And then go here and grab an unofficial balance patch made by a member of the community that fixes a lot of the issues and imbalances in the mod (some general price changes and changes to underpowered and underused units).  And then we'll talk about you installing Hamachi and Xfire and joining up with my group who all play the 3.52 patch.

The metagame is different in each of these games. (I know you didn't ask for it, but I hate DoW2). The metagame in DoW is to expand like a disease across the board and establish holdings that you can defend with your troops. In DoW2, there are no holdings. This is hard to explain without saying why I hate it, but you have to run and capture points as you did in the past, but they have no build-able defense, just something that only slightly slows down the retake by the enemy. Ultimately it comes down to running your melee around capping and uncapping while you amass enough of an army to win.

@Sparda- I do agree that DoWII probably shouldn't have the DoW name. It's just too different (but it isn't sucktacular at least). That said, the main attraction of DoWII is the very tactically focuses battles and the "sneaky-sneaky" things about it.

@Pfizzyhead- yeah, no default static defenses is a bit of a bummer (even though the SM Techmarine can call down turrets; I don't think that counts since it's commander dependent).

There is a Listening Post mod on the community.dawnofwar2.com mod section BTW, if your interested in it. I saw it while browsing through there.