By US political standards, Lieberman is a centrist. Gephardt and Edwards are left-leaning, Kerry is left, Dean is solidly left, Kucinich is far left.
Bush is conservative but not far right. McCain was more moderate than Bush, but many of the 2000 Republican primary candidates were more conservative. Most people from abroad seem to judge him entirely on the Iraq war, which is certainly important but not the only defining moment of his presidency. One of his favorite governing strategies has been to co-opt popular Democratic initiatives and push them through Congress with a Republican tilt. This takes issues away from the Dems, but Bush has upset many conservatives in the US with his enormous spending programs (No Child Left Behind education bill, farm subsidies, Medicare expansion, etc.).
Nondefense discretionary federal spending has been exploding under Bush, so liberals who call him a tightwad starving essential government services have absolutely no leg to stand on. This rampant spending is creating huge budget deficits, but most of the Dems would repeal the tax cuts in order to spend them, not meaningfully reduce the deficit.
Spending increases under Bush (2001-2004), by department:
Health & Human Services: 21.4%
Housing & Urban Dev.: 6.1%
Veteran Affairs: 29.4%
Total Outlays: 15.6%
Also, as mentioned in the 50% Dem / 50% Rep thread, the electoral college makes it very hard for more than two candidates to compete. States usually assign electoral votes on a "winner-take-all" basis, so third-party candidates must carry a plurality of votes in a majority of states, which is very difficult (note that if no candidate gets a majority of electoral votes, the House of Representatives selects the President, each state delegation getting one vote). This effectively shuts out third parties, and given that the public pays most attention to the presidential race, makes life difficult for third-party Congressional candidates. There is one independent Senator who is aligned with the Democrats, and a few independent / third-party Representatives, but they make up a tiny fraction of federal officeholders.
Unlike Europe, there are no coalition governments formed in the US, so the two major parties have no incentive to cooperate with the minor parties. Third-party candidates are rightly seen as "vote-stealers" who siphon votes off major party candidates. Nader (Green) almost certainly cost Gore the White House in 2000, as Perot (Independent) probably did to Bush I in 1992. As others have mentioned, the parties only run one candidate to maximize their share of the vote.
[Edited 2004-01-10 20:11:35]
Keynes is dead and we are living in his long run.