A hydration pack can come in all sorts of shapes and sizes. Some are simple bladders, where as others act more like a back pack with a bladder incorporated. Of course, the main thing that needs to be decided is what the pack is going to be used for. A motocross rider might want a simple and compact hydration pack, where as a hiker might go for the back pack approach.
The best sellers tend to be around 1.5L, but do not let this sway your decision. Packs come in all sorts of sizes but generally between 1-3L. Once you have decided what it will be used for, then you will be able to determine which size you really require.
#2 Bladder Valve
One thing that you will want to look for is a hydration pack that has a bite valve incorporated into it. A lot of people underestimate how important this really is! Having a bite valve will mean that the liquid inside only comes out when the valve is bitten. If the item does not have this valve then you are risking losing all of the liquid inside.
#3 Extra Space
Hikers in particular will want a pack that offers extra space to carry their essentials. The likes of food, tools, mobile phones and keys are all simple items that need to be carefully stored away. If this is the case then you will need to look at a bag that has extra storage space.
You can quickly determine how comfortable a hydration pack is likely to be by the look of the padding and straps. Extra straps tend to take the pressure away from the main points where the bag touches the body. This is not an essential feature for everyone, but people wearing these bags for hours on end would certainly benefit.