I've finally finished the main quest line of Oblivion (only 159 hours in on steam, not including my presteam purchase that was probably similar) and likely to be moving on to Skyrim soon. My experience with Morrowind and Oblivion is that they both had an expansion that was rather invasive as to how you'd have experienced the game had you played it without it installed. In Morrowind this was the "island to the north", where all the rumors were automagically about the island to the north, and only rarely would anyone talk about anything else. In Oblivion it's the Knights of the Nine where they kill off the members of a chapel and that's all people talk about anymore (you don't get to experience the chapel if you start with it installed). Are any of the Skyrim expansions this way? (note: it's not that I dislike these expansions, I just preferred to wait to activate them)
Sort of. If you activate the Dawnguard DLC, random vampire attacks will occur all over Skyrim, often killing many non essential NPCs. However, there are mods you can install to prevent such attacks.
The hearthstone dlc doesn't add much invasive things. The Dragonborn dlc may add some dialog, but not everyone will talk about it. However it also triggers random attacks, but only targeting you.