Is fleece the ultimate mid-layer? It's such a versatile material it's hard to escape that conclusion.
Fleece is warm, comfortable, fast drying, breathable, durable - all the properties you'd want from a midlayer. It also comes in a variety of weights. Heavier fleece jackets are fine in cold weather but for much of the year they're too warm, especially when worn under a waterproof as they so frequently are.
But lightweight fleece (500 grams or less per garment) is flexible enough for year-round use and is ideal for the layering principle: two or even three thin layers are far more versatile than one thick one. It's also particularly good for backpacking as it's low in bulk and doesn't add much weight to your load.
Lightweight fleece garments can be worn as base layers in cold weather; indeed some are designed to be worn this way and have a wicking treatment applied to the fabric. Fleece for next to the skin wear should fit quite closely for maximum efficiency.
Lightweight fleece designs tend to be fairly basic, usually pullover - style with a high zipped collar. A few garments have full-length zips and/or low collars and button necks. Traditional shirt styles also exist. Good features are a high collar for warmth with a long zip for ventilation and a long length and non-elasticised hem so that the garment can be tucked comfortably into trousers. I like sleeves that can be pulled up for ventilation too. Pockets can be useful, but only if they're big enough - some are so small you'd be hard-put to squeeze a compass in.