The expansions were incorporated into the game with the release of TTK, and as such, they begin and go through the low to mid levels of the game for the areas they are available on. The Dark Below missions begin on earth, and so begin at lower level (11, I think?) and the House of Wolves begins when you can get to the Reef, if I remember right. You missed out on the 'challenging' version of the DLCs for 'experienced characters' by waiting until they were incorporated, sadly, and you can increase the difficulties, just not without leveling up your light level in TTK, as the difficulty increase is tied to light level, and if your light level is too low, then it'll say it's impossible. If you want to experience the joy and challenge of the dlc levels at the proper power level, now, your best bet is to create a new character and play through the story that way. As far as the light level thing, it appears (my opinion only) to me that what used to be a -4/+4 level difference is now a -40/+40 light level difference. Things that are 40 or more 'light levels' above you do major damage to you and are immune to your attacks. [Note, I may be off by 10 or so, I think it's 40, but I've not thoroughly tested it]