If you care about noise and performance at high ISO, than what you care about is sensor size. Or better the ratio between the sensor size and the resolution (mm²/MP). Additional megapixels can be harmful here.
The bigger the sensor, the larger is the area corresponding to one pixel, the more light it can get (assuming the same level of technology used), the better signal-to-noise ratio. Speaking about quality, there is less need to remove noise (in-camera or in post-processing), and thus more details are preserved.
Most digital SLRs have a sensor of approximately APS-C size, 25.1 × 16.7 mm. Cameras of the Four Thirds system have a slightly smaller sensor (18×13.5 mm), and "full-frame" cameras have a slightly bigger sensor (36×24 mm).
Compact cameras' sensors are an order of magnitude smaller (typically 7.7×4.6 mm). The difference is usually is related to different in-camera processing approaches used in different camera models. Some may do more noise reduction and produce a smoother image with fewer details, and the others may decide to do less noise reduction but preserve the details. Megapixel inflation is particularly harmful on compact cameras.
The downside of the bigger sensor is, apart from the price, that it requires bigger and more expensive lens. So the camera itself tends to be bigger and heavier. You can get the best of the two worlds (the big sensor and the small camera size) if you choose one of the new mirrorless cameras. Sigma DP series, Olympus E-P series (Pen), Panasonic G series, Sony NEX, Samsung NX are some of such cameras.
