Important features for a hiking GPS:

What features are important for a particular use are a very personal thing. And.. The features needed for hiking are a bit more extensive (and a bit different) from those needed for automobile navigation use. Below are my "essential hiking feature list" of GPS receiver features.
1) "Map" screen (as opposed to just a numerical lon/lat position fix): Makes it much easier to see where you are relative to reference waypoints. 2) WaterProofness: Unit should be rated submersible to protect it from getting wet. 3) Long battery life: Keeps you from having to carry so many batteries. Some units with 2 batteries have longer life than some with 4.4) Built in Maps: Not absolutely essential, but big help in orientation and estimating distance to nearest roads. We now consider that any new GPS purchase should be one with mapping capability. 5) Route capability: Twenty Route capability is pretty standard, some have 50. Make sure you get one with at least 20 routes. 6) Waypoints: 500+ user entered waypoints are pretty standard, some can store many more. Make sure you get one that can store at least 500.. 7) Multiple Datum capability is pretty standard: Make sure all the datums you will want to use are included in the GPS you select. 8) Fit nicely in your pocket and lightweight: This is a very personal thing, but you should consider it when you are thinking about hauling a unit around in the wilderness. 9) Bearing to next waypoint: Almost universal, but essential to hiking use. 10) Screen Size and your ability to read it are important especially if you have problems with viewing fine detail. B&W screens are fine for hiking and many color screens are hard to read in direct sunlight. TransReflective Color screens are the exception.11) 12 channel parallel receiver system: Needed for best reception in difficult terrain and tree cover.