It's the same thing that seperates every other component from great to not-so-great: the power supply and output stages. The better cdp's are usually more expensive because they use premium parts for power supply, filter caps, voltage and current regulation stages, isolation of power supply for signal and operational (LED's, mechanical, controls, etc), shielding, output devices, and overall circuitry design of the output stages.
You can't really go by the type of DAC or drive mechanism alone to judge whether or not you're getting a quality product. Most low end cdp's that vary in price will use the dame DAC for example, but the price difference is based on convenience features. The difference in sound, for example, of the Marantz 63 and the 63SE is night and day - all due to the power supply upgrade in the 63SE.
You can't really go by the type of DAC or drive mechanism alone to judge whether or not you're getting a quality product. Most low end cdp's that vary in price will use the dame DAC for example, but the price difference is based on convenience features. The difference in sound, for example, of the Marantz 63 and the 63SE is night and day - all due to the power supply upgrade in the 63SE.