This is not a spoiler: I always got the feeling that Valyrian steel wasn't really "magical" in the usual sense of magic swords. It's just really high quality steel: very light (which makes that huge two hander usable), very sharp and stays that way. It doesn't make the user any more skilled or powerful, but the owners are almost always great knights and lords so they're usually the best swordsmen anyway. The Valyrians used magic to make it, but that's a bit of a different thing. Lord Marmot (leader of the Night's Watch) giving Jon his Valyrian sword was a huge deal for that reason; it was his family sword and he's handing it to a bastard and new member. Reinforces that "there's something special in store for you, boy" message.
Incorrect! Nerd alert. Valyrian Steel is one of the few materials that can be used to kill a White Walker, right?