There is a black magic in the art of mastering; using different mastering engineers will yield drastically different results from the same source material, there even when they utilize similar techniques and equipment. A “good” or “great” sound is very personal to every set of ears, medicine and is at best elusive.
The variations in flavor show up especially when it comes to the final application of compression – which is a barely predictable beast, even in non-mastering applications.
For the compression component of his or her signal chain, a good mastering engineer will have a stellar, and quite usually, stupidly expensive piece of gear in the signal chain, most likely utilizing some sort of vacuum tube-driven voodoo and highly regarded brand name circuitry. A good compressor will keep the stellar clarity of the source material in tact while adding its own signature sound, even when applying the most insignificant amount of gain reduction.
For the gear head mastering engineer, the compressor may have even been built from the ground up, or hot-rodded from an existing unit – and they can get quite irritable when you wonder aloud if you wouldn’t rather have something you recognize. In any case, it is better to have a mastering engineer who is truly proud of their gear, and has invested time and money into it, rather than someone who just plucks something off the shelf.
This is not to say that there are not so-called mastering engineers who dump a lot of cash into a great piece of gear and use it as their only business card for generating business from poor suckers. When browsing through the list of equipment, no matter how exciting, keep very strongly in mind that even a stratospherically expensive unit in the wrong hands will completely screw everything up for everyone involved – leaving your delicately balanced mix in the hands of a caveman with an analog mixing desk would most certainly be a worse option than telling your brother to “be gentle” as he mashes everything into shape in Pro Tools. Even if you see a hot compressor in the rack, make sure that your mastering engineer’s ears are as good as their equipment.