It appears as though creatures are treated as half cover for the purpose of area of effect abilities, such as fireball or cone of cold.
However, this shouldn't be the case, since creatures only grant half cover against attacks, rather than all effects.
Edit: Apparently AoEs are also affected by cover granted by creatures? Though you could easily put some AoEs in the air so as not to have creatures gain cover. Still, something to think about?
It appears as though creatures are treated as half cover for the purpose of area of effect abilities, such as fireball or cone of cold.
However, this shouldn't be the case, since creatures only grant half cover against attacks, rather than all effects.
Edit: Apparently AoEs are also affected by cover granted by creatures? Though you could easily put some AoEs in the air so as not to have creatures gain cover. Still, something to think about?