The first adventure from LucasFilm showed that Sierra wasn't the only game in town.
You controlled Dave, whose girlfriend Sandy had been abducted by Dr. Fred, and two of six other teenagers from his high school, whom you were allowed to select based on your preferences. They all had different skills, from photography to general electronics repair, and these affected the types of things you could do to save the day. Instead of hunting for just the right word to type, as with the Sierra adventure games, you had a much more streamlined point and click interface.
Being allowed to control three people instead of one was a massive innovation, as were a number of other things besides the aforementioned interface. There were about five different ways the game could end, depending on how you dealt with the Purple Meteor. Controlling multiple characters also provided the opportunity to put you in situations where you had to use more than one character to achieve certain goals, such as having one character push a secret button of some kind to open door while another went through it, or having one character turn off the power while another worked on something electrical without being killed. Truly, Maniac Mansion expanded the possibilities for adventure gaming.
The graphics, although primitive, had a distinctive style, and the interface was the first that could be solely mouse driven, with a set up of fifteen command phrases that you could use to piece together basic sentences with any objects you found and the environment itself, rather than a text entry system.
The sound, of course, was also basic, but Maniac Mansion featured a handful of the most memorable melodies in adventure gaming, most prominently the highly entertaining theme song.
For all of these reasons, Maniac Mansion should be fondly remembered as one of the high points of early adventure gaming, a game that really started to open things up in the nascent genre of adventure games. If you can scavenge a copy somewhere, this is a must play.