A good floor pump is an essential piece of equipment for any cyclist. When shopping for a floor pump, there are a few important characteristics to look for.
The ability to take a beating and keep on pumping is critical to a floor pump. These things tend to get banged around, since their design is inherently top-heavy and you're applying a lot of force to them.
The seals that keep a pump functioning properly are especially critical, since inflating a bike tire to 100+ PSI isn't a walk in the park. Without functioning seals, a pump is worthless (although most floor pump seals can be replaced).
A great floor pump should be easy to use. It should balance well on its base and shouldn't wobble during pumping. The handle should fit well in the hands, and the user shouldn't have to bend over and contort to use the pump.
Small pumps are great for carrying with you on long rides, but a floor pump should be big. Larger floor pumps have larger barrel capacity; they can hold and deliver more air per pump. That decreases the amount of time you spend pumping and makes floor pumps the undisputed champions of filling tires fast.
Hose Length And Durability
A well-made floor pump needs a long hose. Some floor pumps feature short hoses that force the user to crowd up to the wheel, creating instability and increasing the likelihood that a slip will cause your pump to tumble, bending your valve or damaging your pump itself.
Hose durability is also critical. No one wants to spend their time maintaining their pump when they could be riding, so a solid floor pump needs a very durable hose with excellent seals.
Ease Of Attatching/Detaching Hose
You've probably used a poorly-designed floor pump hose. They can cause huge problems, and it usually comes down to the difficulty of attaching and detaching the hose from the valve.
This is a critical aspect to a good pump, and a quality design will make it easy to attach and remove the hose from a valve without skinning your knuckles or banging yourself against your bike.
Some floor pumps don't have a gauge, which doesn't make much sense. If you're riding more than once a year, you want a gauge on your floor pump. A gauge shows you the PSI and lets you hit the target number that's written on side of each tire. Look for a large, easily readable gauge. Some cheap pumps will have inaccurate gauges, which is another reason to invest in a good pump.
While a good pump shouldn't require much (if any) servicing, a great pump can last for 20 years. Over that sort of time span, rubber seals are likely to break down and need replacing. A good user-serviceable floor pump will allow you to access and replace these seals.
A great floor pump nails all the above categories, but some go above and beyond with extra features like accessories and replacement parts.