A quote from Ninki's Regulus guide (okay, it's 1.2, but the basics - except HoL and stuff - are the same, right?):
Regulus is also great against Sedna and QoT because of the heavy AoE damage he does with mines and mark. Although Sedna can heal mark away be heal III, at least you'll force her to use a heal. The heavy AoE damage means that you'll give them a lot of trouble trying to heal themselves and their allies at the same time. Mark also pretty much destroys a QoT.
Dunno, I'm always happy when the enemy Reg concentrates on me (either Sed or QoT). It shuts him down completely, while I just have a bit of trouble - not critical. I really cannot take the mines thingie seriously, since every Sed/QoT worthy of playing this game will consistently minesweep with 350 gold/200 mana minotaurs (shit happens, everyone steps into the mine field from time to time, but it's really infrequent with Sed/QoT). It seems the guide implies that Sed or QoT will always have low hp, and, as such, should be a top-priority target. That's certainly not true, unless someone plays pure speed Sedna.
I'm always amazed when people go berserk following QoT or Sedna... I mean - why? Following a Reg, a DA, a TB into the double towers I can understand. They are squishy and have no safety skills (except Pacov's Fire TB that has both Shield and Heal, of course). I've also noticed it happens a lot to returning veteran players: maybe during 1.2 era Sedna and QoT were played differently, with less emphasis on hp-stacking because of HoL?