I think ADOM needs some more weather effects. At present, there's only rain, which is pretty easily subverted by appropriate gear, and is fairly uncommon. A few extra weather effects added in would give the game more flavour, and maybe provide some added benefits for classes like rangers/druids (or survival skill) who have a number of otherwise fairly useless wilderness class powers. Some suggestions might include:

Snow: Movement in wilderness takes twice as long. PC suffers occasional cold damage. Drakelings suffer reduced speed. For added awesomeness: green wilderness tiles get changed to white. Wilderness water squares freeze over (WDC is inaccessible).
Lightning strikes: PC can be randomly (rarely unless doomed) struck by lightning for massive shock damage + equipment destruction.
Acid rain, rain of fire: PC takes acid/fire damage and suffers acid/fire damage to inventory
Brush/forest fires: PC takes fire damage while in a plain/forest. Alternatively, the fire is a wilderness encounter that can be evaded. If evasion is unsuccessful, the PC is placed on an empty map where they take fire/smoke damage until they escape. For added awesomeness, after a square is subject to a fire, its type is changed from forest to grassland (or to a new type of square, such as badlands).
Solar eclipse: Wilderness and all wilderness encounters are shrouded in darkness.
Storm of raw chaos: PC is corrupted with each step.
Drought: Food consumption in the wilderness is doubled. Survival checks fail much more often.