What does the Spanish word mortal mean when used as slang? Does it have a positive or negative connotation towards the thing being described?
In informal conversation, it's roughly equivalent to 'terrific', as in very good, awesome
'¡El recital estuvo mortal!'
meaning that it was really good, that I enjoyed it very much.
In Spain, and as far as I understand, it has a powerful negative connotation. For example:
La reunión fue mortal.
The meeting was awful.
However, and as it sometimes happens with other words, it may as well be used with exactly the opposite meaning. I haven't ever heard it that way, though. Wheat I have heard, as @Laura points out, is de muerte meaning "extremely good" or "awesome":
¡La comida está de muerte!
The food is awesome!
It can also mean "extremely funny" in Spain.
La broma fue mortal
The joke was extremely funny