I bought the game when it first released, and then pretty much never played it. I was thinking about jumping in but felt a little discouraged when I saw how much dlc there is.
What's important?
You mean as far as DLC? Depends on your playstyle, really. Shadows of the Hist and Horns of the Reach are probably the most ignorable; they both add two group dungeons so unless you're SUPER into that, you don't have to bother (and there's plenty of base game group dungeons and trials). Imperial City is mainly group content, so again unless you're into PvP or group PvE, not much there.
Morrowind and Orsinium are the two large expansions, each about the size of a full zone (if not a little larger), complete with main questlines, side quests, delves, world bosses, etc. If you're into general solo PvE stuff, those are what you probably want--but conversely there's no PvP stuff. Clockwork City is another general PvE DLC that adds a mini-region about the third the size of a normal one.
Lastly, Thieves Guild's and Dark Brotherhood's importance really depends on your playstyle. Do you WANT to join either guild? Otherwise, while they both add mini-regions that are extremely pretty, there won't be much for you there.
It should also be noted that while each DLC is self-contained, there has been a number of story threads brewing in the background from Orsinium to Dark Brotherhood to Morrowind to Clockwork City.
