A broader selection of bits is available with 1/2" shanks rather than 1/4". But for certain light jobs, light routers are preferred because of their lightweight and ease of operation with one hand. As a general rule, the deeper the inside opening or overall length of the collet the better. Good collets also have more lineal splits to grab the bit more uniformly around the shank. And, collets should be made of tempered, hardened steel. Test them with a file. If it makes a cut, the collet is not tempered, but it is soft which is not good.
Large routers up to 3.5 HP are available for industrial/continuous use. Spindle locks and self-releasing collets (pulls the bit out as the nut is loosened) are nice features, but not found on all routers.
The switch should operate easily and, preferably, from the handle or it should be located within reach without needing to remove the hand from the knob or handle.
The design of the base offers more design choices. Some are not completely round but have combined straight and circular edges. This may allow for working closer to a vertical obstruction. It may also make following a straight edge easier. Meanwhile, large round bases have more surface area supporting the router and can bridge large openings. It is important to consider whether there is a provision for interchanging various styles of sub-bases. For general shop use, it is also important that the base have sufficiently threaded sockets that allow safely mounting the router inverted for table use.
The depth of cut adjustment and locking clamp should be quick and easy to operate with a fine adjustment provision. A plunge router intended for use in a router table should be equipped with a depth control knob for easy adjustment or this accessory should be available.
Other important features include overall quality construction, good bearings, durable brushes, and new electricals such as soft starts and the variable speed option. Also check the availability and mounting convenience of template guides, edge guides and the adaptability of the router as a power source for other machinelike accessories such as router lathes, carving machines, or other special router driven joinery devices.