Sealed batteries don't have loose, liquid electrolyte (acid) in them; instead, it's absorbed in a thick fiberglass mat and is thus immobilized. This mat looks like fluffy cardboard when it's not in a battery.
The good: there is nothing to spill when the battery tips over, and very little to spill if it cracks.
The bad: with the immobilized acid, the only ion movement is by diffusion. So you can get stratification which leaves part of the plate in a weak acid solution and part in a strong solution. Ideally, all areas would have the same concentration so the entire plate can work for you. Also, cold performance is theoretically worse because of the slow replenishment of ions at the plate surface (they get 'used up' and need to be replaced). IIRC I could start the K75 down to about 5°F with the flooded battery but the sealed one was only good to about 10 or 15.
Due to the chemistry involved, sealed batteries usually need to operate at a voltage of about 13.2 volts, which is probably why they are so famous for eliminating ABS faults (by keeping the voltage higher during cranking). Ordinary (flooded) batteries are usually designed to have a voltage of about 12.6 to 12.8. You can get a flooded battery to maintain a voltage of 13.2 but it's not really good for it and performance suffers. With higher voltages, the charging force (difference between the resting voltage and the applied voltage) is smaller and you can get into situations where the battery might not get fully charged. This is kind of complicated but the uncharged portion becomes permanently unusable over time.
The reason for the higher voltage is that sealed batteries are designed for recombination, where the gasses given off during overcharge recombine to form water in the cell. Without this, the battery would dry up quickly. The batteries still can dry up if they're not charged very carefully, and it is probably a major factor in the life of any sealed battery, especially in an unregulated environment.
This recombination is achieved by all kinds of mechanisms, but it needs a higher voltage to happen. Basically the designers balance the active material to make sure the positive plate gasses first (giving off Oxygen) and also make sure that there's a gas path to the negative plate. Since the negative is now polarized (brimming with ions, basically) it'll re-form the water in an exothermic reaction. So the side effects are: more heat generated, and less ability to cool off (since there's no loss of hot gases, like in a flooded battery, and no internal convection).