I guess I'll try to explain the game for those of you who haven't played it.

There's four prologue tutorial chapters (which help but don't really explain things in depth as much as FE 7 or 8 do). In one of the chapters you have to sacrifice your characters in order to let the Marth and the others escape. It's a story event where you bring a character to a fort, they dress up as Marth, and run away, getting killed after the chapter ends. They've added a new Cavalier named Frey though so I guess it kind of implies that he was the one sacrificed in the original Fire Emblem.

There's also extra chapters and characters you can get if you have less than 15 characters (or 5, or you killed Tiki and don't have the Falchion) at certain points. I haven't done any extra chapters or gotten any extra characters though, except for Norne the Archer. I resetted when I got her though since I thought Gordin would be with me even after he died in the prologue but he didn't show up.

There's also the reclass system, which allows you to change characters from Clerics to Pegasus Knights and Fighters to Dark Mages (a new class that wasn't in the original, but they're basically just Mages since they still use Anima magic like Fire and Thunder since they didn't add Dark or Holy Magic). The characters' stats also change according to the classes they're changed to. Note that there are restrictions such as Hunters being unable to change to Archers and males unable to be changed into Pegasus Knights. Marth, Thieves, Manakeets (or Mamkuts), and Ballisticians can't change classes either. Also, they've added classes from the other Fire Emblem games like Berserkers, Warriors, etc. to Fire Emblem Shadow Dragon, which'll hopefully make people stop thinking that characters like Barst and Darros are useless. Myrmidons have also been added. Nabarl and Radd have been changed from Mercenaries from the NES/SNES Version, to Myrmidons to the DS Version. Astria and Samson are both still Heroes and not Swordmasters though.

As for the gameplay, I feel it's easier compared to Fire Emblem for the SNES. Shiida now comes equipped with a Wing Spear which is good against Armored and Mounted Units. You can now buy and sell things in the preparations menu as well as use stat boosting and upgrading items. Speaking of upgrading items, there's only one now which is the Master Seal. I'm assuming they did that because you could cheat with the items like change a Cleric to a Pegasus Knight, upgrade them with the Elysian Whip, then change them to a Bishop, basically using an Elysian Whip to upgrade your Cleric into a Bishop. All of the unique class upgrading items are now Master Seals, so don't worry about class upgrading items being rarer now. There's also Bullions now (think Gems from the previous Fire Emblem Games) which come in Small, Large, and Extra Large Sizes, the Small one selling for 10000 Gold and the Extra Large for 30000, which really helps out with money. There's also the grid system with blue squares showing where you can move, red showing where you can attack, etc. The battle window now shows your hit percentage and the damage you and the enemy will deal. There are also some other changes like Ellis being unpromoted, Samson being at level 10 instead of level 1, Hammerne having 12 castings instead of 3, the Falchion having more attack power, and unpromotable units being able to reach level 30. Marth can also talk to newly recruited units if he wasn't the one who recruited them.

There's also new bonus features like the story viewer, (it shows all cutscenes, character conversations, etc. There's also separate cutscenes to view for each character you can sacrifice in the prologue, so it's best to choose someone different every time if you're a completist) music player, Wi-Fi Battling, Wi-Fi character loaning, and Wi-Fi shops (haven't accessed any of the Wi-Fi stuff). Overall, it's a really great game for both newcomers and veterans of the Fire Emblem series and the upgraded graphics, upgraded soundtracks, new soundtracks, and all of the new content make it a welcome addition to any DS game library.

Sheesh, I'm starting to sound like a game reviewer. Also, I'm guessing it doesn't have Fire Emblem 3 included since Nyna actually proposes to and marries Hardin in the end, which would result in Hardin not falling for Gharnef's trap, stopping the whole conflict that FE3 revolved around.