The film raises issues of sexism, racism, political corruption, alienation and the currently ‘live’ issue of the wide availability of guns in the US (although Travis actually obtains his guns illegally).

I have written at length about its ideological dimensions
http://www.derekmcmillan.com/taxidriv.htm

It is a really powerful film. It has one of the most quoted sequences in film, "You talking to me?" which is the fantasy of the central character and narrator - Travis Bickle, the would-be assassin who ends up a hero by mistake. There is a strong portrait in Travis's part-pathetic and part-threatening attempts to fit in to normal society.

The dark portrait of "Hell's Kitchen" may not be accurate - anyone who lives there would dispute it - but it does show how this society looks from Travis's point of view.

I have posted this in hope that other people who have seen it will want to comment.

Paul Schrader studied the dairies of a would-be assassin, Arthur Bremer, before writing Taxi Driver. In May of 1972, Bremer attempted to assassinate presidential hopeful, Alabama Governor George Wallace. As a result of the shooting, Wallace was paralyzed from the waist down. After Bremer's arrest, An Assassin's Diary was published, chronicling his plots to kill not only Governor Wallace, but also Richard Nixon.

Gore Vidal argues that the diary was actually written by a former CIA operative, E. Howard Hunt. We know from the Nixon tapes that he asked a Charles Colson to arrange for Hunt to plant documents on Bremer in order to distance Nixon from the assassination attempt. The diary was actually found in Bremer's car the day after the shooting of Wallace. The important point is that the diary suggests that Bremer was also targeting Nixon. This got Nixon off the hook as he was the main person to benefit from the shooting. Wallace had plans to stand as a third candidate in the 1972 presidential election. Polls suggested that this would let in George McGovern. E. Howard Hunt was later sent to prison for organizing Watergate. It was later claimed that a Nixon supporter was providing money to Bremer before the assassination attempt.

http://www.spartacus.schoolnet.co.uk/JFKbremer.htm