none you will get an uneven cut,and the grass will clog up the undercarriage of the mower,and you will picking grass out more than you will be mowing.Also, if you leave the grass under the carriage ,you have a great chance of propagating fungus,which you can transfer to your lawn,and damage your lawn.It also dull the blades.Mow dry.
A cylinder mower gives the finest finish to any lawn but its never a good idea to cut wet grass. Most mowers will clog up every 2 minutes. If you do have to cut wet grass get a 'swish rod' or a long light pole and sweep the grass over and back sideways so the dew comes off the grass and sits on the soil allowing the mower to work better. If your using a rotary mower and want a good finish you'll have to cut it twice for a good finish (once up and down, once across the lawn)
None, not only will wet grass clog, but it can also dull your blades, no matter if you have a reel, line, or rotary mower. The only way to mow wet grass is with a brush hog, and that isn't pretty, nor is it feasible most likely. Just wait a few days until it dries out a bit, but if you have to mow, select the highest setting you possibly can.
An air cushioned mower (flymo) will cut grass when it is wet but it is better to let the grass dry before cutting it or follow the suggestion about removing the moisture first.On our cricket square we use a brush which is pulled over the grass first which disperses the moisture and enables us to cut the grass.