The problem is that most hobbyist or professional woodworkers may have limited shop space or a limited budget that prohibits the installation of a full-scale dust collection system.
The first feature to look for when choosing a woodworking vacuum is air flow. If the vacuum doesn't have a large enough motor to pull in the majority of the sawdust that your woodworking tool creates, it isn't much good. Compare the CFM (cubic feet-per-minute) ratings between different units. Typically, a motor with a higher amp or HP rating will have a higher CFM, but not always.
Second, check to see that the hose is large enough to handle your biggest woodworking tools, and that you have the ability to connect to all of your power tools. You may need to buy an accessory kit to match all of your power tool ports.
Fourth, try to find a unit that isn't overly loud. Some models are powerful, but require the user to wear hearing protection just to clean up the shop. While you may find this acceptable, it can be a hassle at times, particularly when you just want to clean up a small area.
Vacuum systems designed for woodworking typically have a few additional features that are quite helpful. The first is a switch that can be set to come on whenever you turn on the connected woodworking tool. While this isn't a required feature, it is one that you'll certainly appreciate from the first use.
Another quality feature to look for is a HEPA filter system. This type of filtering system should help to reduce the amount of dust that gets past the filtering system and pushed back into the air through the vacuum's exhaust system.