Assuming that you're an adherent to the principle of layered clothing, midwear is that ubiquitous term which covers everything which isn't a base layer or shell garment. Before the advent of pile and fleece fabrics, that would have covered natural fibre garments made from wool or cotton, but these days, synthetic rules supreme, with the vast majority of midwear garments made from brushed polyester fleece.
So what kind of things do you need to bear in mind when browsing the stores for a fleece top?

The Cut
If you're after maximum efficiency, a close but unconstricting fit is essential. Having said that, most people buy fleece jackets and pullovers with an eye to fashionability as well as performance, so they'll end up with a somewhat looser fit.

For general use, what you have at the end of your sleeves is really down to personal preference. But if you're likely to be doing anything serious on the receiving end of bad weather, it's best to opt for Lycra bound or elasticated cuffs rather than rib knitted ones which tend to hold the water and feel uncomfortable. Self-cuffing sleeves is another option. Here the material is simply doubled over at the end to form a soft and comfortable cuff which uses the natural stretch of the fabric, and like the rest of the jacket, won't take up lots of water in rainy conditions.

Bottom Hems 
Often use the same form of gathering as for the cuffs - either elasticated, trimmed with stretch nylon, or with knitted cuffing. But quite a few designs employ elasticated drawcord with cordlock adjusters. Getting the gauge and elasticity of that drawcord right is something that a good few manufacturers have failed at dismally. Spindly shock cord with too soft a stretch ends up as yards of "tail" hanging either from the sides of your jacket, or even more ridiculously, from the front.

Given that a fleece top is made to keep you warm, it follows that the hip pockets should do that for your hands. Manufacturers have to balance a line between using fabrics for pocket linings which don't add greatly to the overall bulk around the hips - which is what can happen if the main body fabric is used throughout - and using thin pocket lining fabrics which don't perform well. The best compromise without doubt are the thin single-sided microfleece fabrics, which feel warm and soft to the touch without adding too much bulk to the garment. 

The idea of zipping a fleece into your waterproof jacket is, I have to say, nothing more than a gimmick. It doesn't make your clothing setup any more effective than wearing the two layers separately, and it means that you're paying extra for the second zip in your waterproof. 

Windproof Fleece
There are several varieties of windproof fleece around, including Polartec Windbloc and Gore Windstopper, using a breathable windproof membrane laminated between two layers of fleece. They're effective, but they also have a downside in as much as they can be too efficient if you're working up a sweat. My preferred option for windproof fleece is a separate lightweight windshirt which can be removed when needed. 

Fleece fabrics using polyester from recycled soft drinks bottles are becoming more and more common, and a jolly good job too. The number of these things getting buried in landfills each year is positively staggering, and any efforts to minimise that have to be commended. Ironically perhaps, the fact that those plastic bottles are made to withstand being dropped on the floor means that the polyester from them is actually of higher quality than virgin polyester.
Early recycled fleece fabrics tended to have a proportion of virgin fibres in them - these days the proportion of recycled is much higher, in some cases it's 100%. As the technology for extruding finer and finer fibres progresses, the next development will be microfibre fleeces made from recycled polyester.