Macro lens construction
By definition, macro optics are designed for close-up shooting. If you look at their optical construction, you tend to find symmetry in the arrangement of the elements.
The design towards the front of the lens is typically repeated in reverse towards the rear. This's so that the rear elements cancel out some of the aberrations caused by the front elements. Greater correction is required as the magnification factor increases.
Another common feature is the 'double-helicoid' design of the focusing mount. This gives the large focus range required by a macro optic. Floating elements and lens groups, which move independently within a lens, are also commonly employed to correct spherical aberration, the extent of which varies with distance.
Using floating elements requires a more complex design, which is often reflected in the price of the lens.
Macro lens features
Shallow depth of field aside, a wide maximum aperture on a lens, such as f2.8, means it's capable of admitting plenty of light to help keep shutter speeds high.
This shows the distance at which a lens focuses in feet and metres. On macro lenses it often also includes the reproduction ratio, which decreases with distance.
Focus limiter switch
This effectively closes off part of the lens's focusing range, which is useful if you're only shooting close-up and you don't need to focus on distant subjects, or vice versa.
A lens with an internal focusing system will ensure that it remains at the same physical length while focusing, which is particularly handy if your subject is close to the front element of the lens.
Minimum focusing distances tend to rise as the focal length is increased, so longer lenses can be useful when you need to keep a safe distance from a living subject, such as a small insect.
This can be useful if the body you're using lacks a sensor-based image stabilisation system, although if you're using the lens on a tripod for macro work, it's best to turn stabilisation off.