The choice of trousers is quite important. This is highly depenedent on the sort of weather and temperatures you expect to find, but generally I go for shorts first, and for as long as possible, then to light trousers, then to heavy trousers.
The reason I go for shorts over trousers in the summer months is comfort. There are many types of shorts that you can buy with cargo pockets, zip pockets, double pockets and so on. So in terms of stuffing stuff into your pockets, these are just as good as trousers. However, if there is anything more than a little sun, hiking up a high gets me very sweaty. At this point, having a large heat-sink (my lower legs) exposed is a great way to keep cool.
Beware if you go down this route, though, as there are hazards. The first is the dammed burrowing mites that you get in heather. These bite into you, bury their heads, and drink away. You need to be careful when getting them out as if you break off their head the wound may become infected, but they’re not painful.
Likewise, midges (small flying beasties) will have a go at any exposed skin. The first you’ll know is an itchy bump that gets itchier and itchier. You can avoid these guys by knowing where they hang out (forests) and when they come out (dawn and dusk.)
Finally be careful when you stop with shorts on. You can get quite cold quite quickly. So long as you keep your core warm this isn’t going to be a problem, just uncomfortable for a bit. (Keeping your core warm is all about having a great fleece on your upper body, along with a windproof or waterproof). However, if you haven’t got that sorted you can get cold much more quickly if you’re also wearing shorts.
But stand fast that lot, shorts can be the way to go in the summer.
Moving into less warm periods, or during the summer when it actually can still be rubbish weather, light utility trousers are the thing. If you can get the very thin sort that wicks away water, that will oddly keep you warmer than heavier cloth that doesn’t. What happens is that they get wet, the water spreads out, has a bigger surface area, and dries more quickly. So if you’re going through showers and don’t mind being wet for 10 minutes, light trousers are the thing.
Avoid denim like it’s the devil’s own material. It will get wet, then drag you down to a cold, wet, miserable death.
Finally, for very cold conditions, there are heavier trousers with similar wicking features also available. Some of these are even fleece lined, which you’ll love yourself for having the foresight to buy when you put them on in the morning. (Fleece has the weird property of always feeling warm to touch, even when it’s wet.)
Whichever it is, shorts, light or heavy trousers, I always take two pairs. This allows you to go into wet and dry routine, which sounds horrible but absolutely is the way to go for trips of more than one night.