Yes, you have to get to level 20. Promoting characters resets all the characters in that class to level 1, and adds a 75 point war asset to all of your single player files.

FOR EVERYONE ELSE:

I wonder if the DLC is going to be multiplayer based (and free :P!).

I love the multiplayer, however, more modes, would be... even better. And maybe a ladder with actual teams like starcraft. I'd even pay for those. Can't say the same about Single player DLC though.

can someone tell me what promoting does. Do I have to get a character to like 20 or something?

Yes, you have to get to level 20. Promoting characters resets all the characters in that class to level 1, and adds a 75 point war asset to all of your single player files.

FOR EVERYONE ELSE:

I have a system I'm working through in order to get basically all the unlocks, though it's going to take some time. The way it works is that when characters are at level 20 and you get all their customization unlocks,y ou don't get those character cards anymore. When weapons get to level 10, you don't get those anymore. Mods at level 5, same thing.

You can see where this is going.

When my Vanguard is back up to 16 or 17 I'll start farming Silver, and at 20 I'll jump into Gold. I'm going to spend all of my money, at the start, on Recruit Packs, so that I can unlock all the base-level stuff faster. After that it's going to be the Premium Veteran Packages, so I can get all the Silver unlocks faster.

This will take some time, but it seems the most efficient way to do it. And the Katana X is a lot better than any other shotgun at level I, just like the Mantis X is better than the Widow I - weaker, sure, but also faster, and weighing about a third as much.

That doesn't work, you still get Character cards, even if they're at level 20, I've tried that myself. When I was levelling up my Soldier to 20, every other class was already at 20, and I was getting cards for them. 

As for the weapon cards, you're right about them dissapearing once they hit X, but as you go on in the rarity scale, each level has much more weapons than the previous ones, so it'll end up being an exercise of extreme morosity. And I'd disagree that the Mantis X is better than the Widow I, especially if you don't have a Cover Penetration mod. The Mantis takes as long as the Widow to reload and the Widow at least can punch through two enemies with ease, giving you two kills per shot. You'll get much more kills in the end than with just the Mantis.

But did you have all the customization options for those characters already? I submit not!

And the Mantis X is good for higher level engagements in that it carries way way WAAAAAAAAY more ammo, so in higher difficulties where going for more ammo can be dangerous (such as when defending areas as a Soldier or Engineer), you're going ot have definite advantages with the Mantis. Especially because, again, it weighs a third as much.

Probably not, but my Soldier, Vanguard, Infiltrator and Adept do have them. I'll try to see if it makes a difference. 

Well, I took care of the ammo issue with the Spare Shot mod, I can carry 19 rounds, so it's pretty much up there with my Mantis X. The only real advantage of the Mantis is the weight and even then my Human Infiltrator can handle itself pretty well with just +11% recharge time. I agree that with a Soldier or Engineer class, it would be much trickier to defend an area with the Widow, but that's why I usually go with the Avenger X, the Shuriken X and the Carnifex II. My Turian Soldier can handle those and still be at +105% recharge time, so I can handle territory and crow control nicely (Marksman is just a beast).

Try rolling a Salarian Infiltrator - with the viability of both Shield Drain and Tactical Cloak, you come to appreciate recharge a lot more. Now granted I'd still take a Widow of equal level over a Mantis any day, but Infiltrators (that aren't human) can make damn good use of their powers in this game.

I don't have the Salarian Infiltrator as well :-/. The packs have not been kind to me in that aspect. Heck, I'm still stuck with the Human variants in both Sentinel and Infiltrator.