Minor language quibble
This is poorly worded (and, I am working on the text as presented, so this is my best fit):
Hypersmurf said:
PHB p56: An area power creates an area of effect that hits multiple targets or creates an obstacle.
I would imagine there are cases where "hits multiple targets" resolves to just one target (or none!)
Otherwise, you get a strange ambiguity:
XXX creates a line effect that attacks all enemies in a line six squares long.
If there is one (or no) target in the area of effect, the effect is an obstacle.
If there are at least two targets in the area of effect, the effect is ... actually, I don't know. The effect could be either just effecting multiple targets (and not creating an obstacle), or could be effecting multiple targets AND creating an obstacle.
I think what that works down to is that the rule tells how to determine whether or not an effect is an area effect, not a rule saying what an area effect must do. That is, IF an effect targets multiple enemies OR the effect creates an obstacle THEN it is an area effect.
Or, if the text is prescriptive, then it is
very badly worded, as it provides no logic to tell which of the two cases is to apply.