In reply to Rick Atkinson:
Not sure how I missed this one...
The purpose of a base layer is twofold, IMHO:
i) to maintain a comfortable microclimate against the skin, by drawing water away from the skin
ii) to allow that water to be evaporated, so that the sweat can do its job of cooling the body.
To do this, it's a good idea if the base layer wicks sweat from the skin, and spreads it out over a large surface area, to maximise the potential for evaporation.
A base layer that merely moved water away from the body, and beaded up on the face of the base layer wouldn't be very effecitve at allowing the body to regulate its temperature.
The purpose of a mid layer (e.g. fleece) is to trap a layer of warm air close to the body, keeping it warm, whilst allowing the evaporated moisture to pass through the fabric and escape to the environment. Thus, a fabric with an open structure is useful (hence fleece). If the fabric is such that it traps water, this isn't good, since it removes the dead air space, and replaces it with much more thermally conductive water. If the water is trapped, it can only be removed by evaporation, which is likely to chill the wearer.
Since the mid layer is an open structure, the warm air it contains can be easily stripped away by wind, thus rendering it nearly useless. So, we need to protect the mid layer from wind using a shell that stops wind, but allows water vapour to continue to escape. Whilst most of the waterproof manufacturers will claim that their wonder fabrics will do this, the reality is that none of them are really yet up to the job. Which is why you're better off using a wind resistant shell most of the time, and carrying a lightweight waterproof to put on when it really starts to rain. The wind-resistant shell, being unproofed, breathes a lot better than a waterproof. You can choose a wind resistant shell to meet your needs for robustness and wind resistance (as well as design features).
This is a minor change to the long-established 3-layer concept; simply adding a windproof shell as a 'layer 3a' option, with a waterproof as 'layer 3b' option.
Now we need to consider the difference in body heat output between active and static states; this can be as much as a 7:1 ratio. So, whilst we're active, walking uphill, we don't need to wear too much clothing to stay comfortably warm (our 'action layer'). But, if we stop for any length of time, we're no longer putting out enough heat to stay warm. One solution is to add an insulating overlayer to wear on top of everything else (in climbing terms, a 'belay jacket'). Now, since we're likely to be carrying this quite a lot of the time, we want it to be thermally efficient, lightweight and compact, and those requirements are often best met by a down jacket or synthetic equivalent, depending on warmth needed, and conditions likely to be encountered.
So, a typical layering system might be:
base layer
100 weight fleece
lightweight, wind resistant shell
lightweight waterproof shell
synthetic overlayer
This system is versatile, coping with a range of activities and conditions, and allows the items to be selected to meet the needs of the activity and conditions, and be cared for and replaced individually, as required.