Have gone through the other dlc packs i would say this is my favourite one. Broken steel starts of after you finish the game, shame doesnt change the ending. You recieve a message stating your level cap has gone from 20 to 30 With this you gain more perks and also upon reaching level 20 the there are harder enemies on the loose.
The story of broken steel is you continue your battle with the enclave, these missions are more combat focused and you with infiltrate the enclave base and gain the best weapon in he game, the tesla canon. This dlc pack will take you about 4 hours, but zapping enemies with the canon is too much fun. The quests are given to you by the scribes and the aim really is to destroy as much of he enclave as possible before they regroup.
Again this is the best dlc that fallout 3 has to offer, but may be the new one coming out can top this one.