I'm not an expert on Robert Altman's films. I can't say I'm a big fan of his work. However, he was a filmmaker that took a lot of risks on stuff that most people wouldn't dare do. (How many films not involving Christopher Guest have you heard of where they started shooting a film with a general idea instead of a script?) He was unafraid to say stuff that a lot of people thought about but couldn't put into film. This is why it's sad to hear the news about his passing.
A number of Altman's films are staples in film courses and film schools. Myself, I was first introduced via the eight-minute, one-take opening in The Player. Blasting the MTV style of fast-cutting while paying homage to Orson Welles' Touch of Evil opening shot, I was impressed. M*A*S*H came a few months later, but I didn't see another Altman flick until earlier this year.
If you only look at his highest-grossing films and say that's all the worthwhile stuff he did, you're missing a lot. Films like Nashville and Short Cuts aren't popcorn fare where everything is neatly handed to you. Instead, they're snapshots of specific times in the last forty years that don't feel dated today. Nashville isn't just about country music in the Seventies and Short Cuts isn't just about living LA in the Nineties. He focuses on humans going through human things more than the always-shifting pop culture of the day.
While there can only be one Altman, understand that he's not going to be forgotten any time soon. He was always respected among film critics and scholars. Also, a number of younger directors, including Richard Kelly and Paul Thomas Anderson, cite him as a major influence. You can debate all you want about how Anderson's Magnolia and Kelly's Southland Tales hold up to Altman's stuff, but it's hard to argue they were taking cues from a great.