Comfort in a hammock depends on four things: Material, Weave, Spreader Bar (or lack there-of) and how it's hung. Here's what to look for:
As a general rule, natural fibers tend to result in more comfortable hammocks than synthetic ones. This is one area where American made hammocks generally miss the boat. The all-weather Duracord, Sunbrella or even Polyester hammocks are great for leaving resisting the elements and being left outdoors for months on end. But while their great at standing up to the overnight rains and pounding mid-day sun, they're not so great for an afternoon nap. These fabrics sacrifice comfort, softness and stretch in exchange for resistance to rain and mildew.
The weave is perhaps the most important element in determining a hammock's comfort. The ideal hammock has an open weave with a double or even triple spring weave. This weave is found in hand-woven hammocks from Mexico (often referred to as 'Mayan' hammocks) and Nicaragua. In the double or triple spring weave each individual strand of cotton or nylon is woven in and out of the adjacent strings 2 or 3 times in an interlocking diamond pattern. This allows the hammock to shift, stretch and conform to your individual weight and shape, and makes the hammocks extremely strong. Also, the open weave makes them very cool and breathable for those hot summer days (or comfortable nights sleeping in your bed hammock). Most American hammocks are either solid fabric or have a single weave. The single weave is rigid and won't stretch. The solid fabric is equally rigid and also very hot and not breathable at all.
Spreader Bar or No?
Spreader bars are common in American hammocks, but not at all in Mexico or Nicaragua where they originated and can be found in nearly every home. That should tell you something. Ever had a hard time balancing when entering a hammock? The spreader bar is the reason. Not only do spreader bars make hammocks unstable and tippy, they take away the 'cocoon effect' that more comfortable hammocks have, and force you to hang them at a flatter angle to the ground, which is less comfortable.
How to Hang Your Hammock
Good hammocks (those without spreader bars) should be hung in a fairly curved banana shape. As a general rule the bottom, low-point of the hammock should hang just below your waist while the hooks, or 'hammock eyes,' are at roughly head height. More curve will result in a greater cocoon effect while less will make the hammock lie flatter. In general the guidelines above will make the hammock ends angle at about 30%, which provides the ideal incline scientists have found makes you fall asleep faster, deeper and wake more well-rested.