If you dive only in warm water and never touch anything, you can dispense with gloves. In all other situations they prove very useful.
In temperate and warm waters a non-insulating or 2-3mm glove will offer the protection you need, bearing in mind that the best protection is to stay away from stinging things and, in particular, not to handle the coral.
Long dives in warm water make your hands soft and vulnerable to cuts and scratches. Do not run the risk of infected cuts when even the lightest gloves will help prevent this.
As the water gets colder and the gloves thicker it becomes impossible to accomplish any useful work with your hands. The coldest water calls for three-finger mitts or a dry-glove system, but even tying a knot will prove difficult with thick fingers.
The wrist should give a good fit to reduce water ingress, but this can make them difficult to pull on. Many manufacturers offer a model with a velcro fastener or zip to tighten the wrist once on.
To protect the relatively fragile lined neoprene from cuts and abrasion, look for a palm that has a resistant coating or uses a leather-like material to give better wear.