The '04 model has changed significantly from the 2003, so I figured I'd node some of the differences here.
First of all, the car looks different. It has a totally redesigned body, which (I think) looks much cooler than the original (which was sort of boring). Best of all, the new body actually lowers the coefficient of drag to 0.26, which means the Prius retains the title of the 'slipperiest' production car. It also bodes well for fuel economy.
The drivetrain has been overhauled, and now bears the moniker 'Hybrid Synergy Drive.' It consists of a 76 horsepower ICE which uses the Atkinson Cycle, and a 67 horse electric motor. The ICE delivers a wimpy 82 lb-foot of torque, but the electrics offer a beastly 295 lb-foot additional up to 1,200 rpm. Acceleration now is claimed to be equivalent to that of a standard four-cylinder Camry.
Fuel economy has gone up, too. Estimated city MPG is around 50-55; highway is 55-60 (U.S. gallons). All this for the same price (around USD $20K base).
Some of the most functional differences are in the body. The car is now a four-door hatchback, with folding rear seats; its slightly larger wheelbase moves it into the 'mid-size' segment, but it's actually only a couple of inches longer than the previous design.
As before, it comes standard with a touchscreen LCD for car management functions. Available options include a DVD/GPS Navigation system, a slightly nicer stereo, HID headlamps, stability control, theft protection, etc. etc. The neatest new option is the keyless entry system; the driver keeps the key fob in their pocket, and when s/he approaches the car, a sensor 'sees' the fob and unlocks the car when fingers are placed on the handle. Once inside, if the fob is present, the car will respond to a press of the 'Start' button and drive off.
I'm a tad annoyed; Toyota just demonstrated a self-parking feature on the 2004 Prius in Japan, but it won't be available in the U.S. for now (likely for liability reasons). Self-park here means exactly that - once you've pulled up next to a parking place, hitting the button causes the car to measure the spot using sensors and back up into it using the electric power steering to move the wheel. The driver can remove their hands from the wheel entirely. Since I live in the U.S., though, I can forget about this, at least in this car - maybe in a couple of years. Also, there is a 'Stealth' button in Japan-spec Priuses which forces them to operate on electrical power only (at least until the battery becomes unacceptably low) which allows for extremely quiet parking and approach/departure, as well as makes it easy to shuffle around the driveway or lot without having to start the ICE. This feature apparently is missing from the U.S. version as well.
Ah well. Time to start hacking! I already found a source for a night vision CCD camera that hooks into the LCD...heh heh...now, with a linux server and an 802.11b access point to make the car a hot spot...and maybe an AC inverter...