It broke barriers and illustrated what TV comedies could do at their best. Dick Van Dyke Show worked because I think that in many ways the characters were more realistic and believeable (even when they were cracking jokes constantly) than many other sitcom characters that were on TV at that time.
Mary Tyler Moore and Dick Van Dyke were the most realistic couple of TV back during that time. Although it's a shame that even they were relegated to sleeping in different beds.
The show was at its most adventerous (and usually at its best) when it bended the rules of reality a little bit though. Remember the crazy episode when those peanuts came rolling out of the closet? In another episode, Rob is the Sheriff in an old western town. Although I think over the years, ideas like these have gotten silly in sitcoms, DVD Show was among the first to try them. And they were usually successful.
Anyway, check this show out. Until recently, it was a Nick At Nite staple. Although I think it only airs on TV Land now.