One thing that always bothered me about Enterprise-- there were several races we never heard of before (Suliban, the Xindi races) and events that were never heard about again in the chronological timeline of the later (earlier) series. Was this ever explained?
One thing I did like was references to races/systems that would later be destroyed (Malurians).
90% of the races from The Original Series were never seen or heard from again in Next Gen, DS9 or Voyager, either. Virtually none of the cool background aliens from The Motion Picture nor the aliens that made up the Federation council in The Voyage Home nor the aliens from JJ's Star Trek ever crop up in chronologically later Treks.
I like to think those aliens are always there, but always just off-camera.