Since minifigs are so important to vignettes, I figured a brief historical perspective would be appropriate. The first minifigs appeared in 1975, but these were armless, legless beings with no faces. Three years later, in 1978, figs became poseable, and started smiling. This began the Era of the Classic Smiley. For the next decade there was no change in the basic fig; they gained different torsos, utensils, hats, or hairpieces to reflect their jobs as knights, astronauts, and all things in between, but they kept the basic shape and the same jaundiced, smiling face. In 1989 the world of the minifig changed. The new pirate theme brought different face patterns, and even different leg and hand elements, though the classic smiley still held sway in most themes. In 1990, the ghost was introduced as the first speciallized fig. This figure had a speciallized ghost body and a black head (actually black, red, and clear heads had previously existed, but not in true minifigs). In 1992, classic smileys were still the norm, but speciallized face patterns started showing up in themes like town, space, and castle.
1993 brought the beard, and 1994 established the precedent that aliens and robots could have non-yellow heads. In 1995, the skeleton was introduced as another speciallized figure. Also by this point, speciallized face patterns seem to have taken over, with relatively few instances of the classic smiley. In 1997, Native Americans became the first figs with recognizable (albeit caricatured) ethnic identities. These were also the first figs with explictly drawn noses (other figs occasionally had noses implied by the shape of facial hair). These were followed in 1998 by Asian figs in the ninja theme.
Starting in 1999, LEGO started running licensed themes with the introduction of the Star Wars line. Licenses have been a great boon to the diversity of the minifig world, with many innovations driven by the needs of the license. For instance, 2000 brought molded heads with Jar-Jar. Many other molded heads have shown up since, including C-3PO, Greedo, and Yoda, as well as Dobby and the goblins in the Harry Potter license. In 2001, the TIE Fighter set included blank heads underneath the Stormtrooper and TIE Pilot helmets, and the pilots head was brown plastic. This added fuel to the debate of whether human minifigs should be produced in skin tones other than yellow to represent different races. In 2002, the needs of the Star Wars and Harry Potter lines led to the introduction of shorter legs (aka Stubbies), which have since also been used for children.
Also in 2002, LEGO introduced double-sided heads, so the face could be changed by turning the head around. In 2003, LEGO ended a long debate by finally introducing human figures with non-yellow faces, starting with Lando Calrissian. They have since decided that figs that represent real people (i.e. actors and athletes from licensed lines like Star Wars, Harry Potter, Spider-Man, and the NBA) would have realistic skin tones, while other figs would remain yellow. Also in 2003, the NBA/basketball theme led to the introduction of spring-loaded legs and differently shaped arms, to allow the figs to throw a basketball. Now in 2005, electrified figs have been introduced that light up when you press on the head.
How will the minifig evolve next? We don't know, but what is certain is that each new style of minifig will find a use in vignette building.
Caveat: I know this is by no means comprehensive. I've completely ignored any discussion of stickered versus printed torso, solid versus hollow head studs, changes in the molding of the torsos, the evolution of painting (from just the face and chest to the legs, back, and even arms), promotional figs, non-minifig figs (battle droids, Martians, Jack Stone, etc.), or the great variety of speciallized faces, utensils, hats, etc. Many great resources exist for minifigs, including Minimundo, Brendan's gallery of faces, Peeron, and Bricklink.