What makes Blade Runner so iconic?

I'd say a large part of why it's seen as so iconic is because it's influenced a lot of what came after. The movie was released in 1982, and has been referenced as being a huge source of influence for several created works that came later, such as the widely popular Battlestar Galactica reboot that was on TV in the middle of the last decade, as well as games such as Beneath A Steel Sky, the role playing game Shadowrun (sort of a cyberpunk D&D), and even Perfect Dark. There are also the societal implications in the movie that are still very relevant today (human cloning, rights based on genetic origin/disposition, etc.)

You can read a fairly concise synopsis of the film's influence here.

Another way to think of the film as iconic is in its relationship to cyber-punk.

You cannot say, for example, that BR is the first film to be rooted in cyber-punk because it is not really possible to draw a distinctive line delineating where New Wave SciFi of the 60's and 70's ends and cyber-punk--a child (perhaps bastard child :-) ) of the late 70's and early 80's--begins.

However, you could easily make an argument that BR provided a motion-picture platform for the genre/sub-genre that occurred during the earliest stages of what would become the crest of the cyber-punk wave (especially once Gibson's novel, Neuromancer, was released in 1984).

BR is cinema's addition to the rising interest in cyber-punk, along with the visual work of Jean Giraud (Möbius--especially his The Long Tomorrow) and Alejandro Jodorowsky in Métal Hurlant (France, c.1975) and Heavy Metal (US, c.1977) or Otomo's 1988 Akira (or even my avatar, RanXerox, by Stefano Tamburini and Tanino Liberatore!).

All of which built upon the novels and short-stories that came before and arose with the interest in cyber-punk.

BR, although it had wide release, did not do all that well in its initial release. However, it would find new life in the 90's as popularity in cyber-punk continued to rise and was given new life as interest in all things Japan rose, too.

By this point, cyber-punk was unequivocally established and as the surge rose the re-surgence of the film assured its place within the genre.

So, in many ways, BR has always be inextricably bound to--and advanced and was advanced by--cyberpunk.

Perhaps reading Bruce Sterling's essay/preface to Mirrorshades might give you more depth as to why the film--especially in relation to cyber-punk--is so iconic.

NOTE: You can see that the artwork for Möbius' The Long Tomorrow is reflected in the world Syd Mead created for Scott's Blade Runner

Notice in this pic Jodorowsky's influence--from The Incal--is evident in the façade work.

The movie is inspired by Philip K. Dick's novel "Do Androids Dream of Electric Sheep?".

Although a prolific science-fiction author, it is often told that the popularity of Dick's production originates in the fact that he actually writes about universal problems that are independent from the typical science-fiction setting, and to which every human being from every period of time can relate to.

What does it mean to be human?

Why do some people claim to know better what a human is? How some of those people consequently try to segregate - often violently - humans from what they judge as "not humans"?

What are the beliefs of humans about the world, how do they try to make sense of all this?

Why is the mere existence of Life so awe-inspiring?

... might be some of the questions, recurrent in Dick's works, that are also raised in the movie.

The movie illustrates some of those questions ; watching it gives the viewer an opportunity to remember to think about them, and to amaze before the mystery of Life.