I was not a Sierra adventure fan (except Gabriel Knight), but I loved EVERY ONE of the Lucasfilm Games. Why? I think they managed perfection in these four areas:
ACCESS, FUN, CHARACTERS, DESIGN
ACCESS: You fired up the game and could immediately start playing. Most of the games had no or very short intros or cutscenes. Lucasfilm was very strong at telling the story throughout the game. "I'm Guybrush Threepwood and I wanna be a pirate" -> There you go!
Controls were also very easy, I think they were even the first games that you could play through by only using your mouse. That's important, because you could just literally lean back and play.
FUN Maybe it's just me (obviously not) but Gilbert's and Schafer's writing style, that's my idea of fun! Well written, perfectly executed and timed. Often with very few or no words at all. Sometimes Bernard or Guybrush just turned to you and said nothing…all said!
CHARACTERS The teams put a lot of effort in creating unique characters. Why the hell do I know names like Guybrush, Elaine, Stan, Bernard, Hoagie, Laverne, Ben, Bobbin, Manny, Glottis; but I can hardly remember any other adventure game character names. It's because I was emotionally bounded with them. They were mostly struggling with their environment but managed their obstacles often with unorthodox methods.
DESIGN Especially Ron Gilbert and Hal Barwood established adventure game design that is still considered standard today. Brought to fame by Ron's article "Why adventure games suck – And what we can do about it" (http://grumpygamer.com/2152210). This article still inspires me today and I'm throughout checking my designs against it.