Rucksacks

With the current plethora of gear manufacturers there is pretty much a rucksack to suit everyones needs. Whether that be hyper lightweight, max durability or specific features like ski slots.

In terms of what to look for:

The back system

• Should be comfortable above all else
• For winter or alpine use the foam used shouldn't absorb water (i.e. open mesh backsystems are not great when wet snow gets absorbed in to them)
• If carrying larger loads then a frame is a big advantage

The main rucksack:

• The fabric should be durable (enough to stand up to the usage you intend to give it) 
• The fabric should be very water resistant (although very few rucksacks are waterproof, there is a big difference between those that will keep the worst of an alpine thunderstorm at bay and those which seem to wick moisture to the inside of the bag! Even though you should have all your gear in dry bags on the inside of your rucksack it is still a total pain having soaking wet dry bags and a water filled rucksack weighing you down.
• If it has a port for a hose, make sure it isn't going to allow water and snow to be blown straight in to your bag. Also make sure the port is big enough to fit a hose with an insulated sleeve through it.
• It should have simple to use side compression straps that effectively compress the bag.

The lid:

• Should be large enough to fit things of use in to it (some rucksack lids you can barely fit a car key in to them)
• The zip should be large enough to access everything in it.
• It can be really useful to have an extendable lid so that when carrying large loads everything fits in easily, but once things have been removed it can be cinched down to create a smaller capacity bag.