I think he *may* have meant "defined" as in restricted or limited?
Quicksilver has a pretty defined and "limited" power - he can run really fast. Sure, you can expand upon that to include additional powers, but that's his power in a nutshell. In any circumstances where he can't use those powers, then he's limited and if Ultron or Vision can also move fast, then he can be easily challenged.
With Scarlet-Witch, I have to presume that in his head, the movie version of Wanda has a defined and somewhat "limited" power set. Maybe she needs to be emotionally stable, calm and rational in order to exercise her powers in any sort of controlled manner? Maybe she needs to do the bizarre "calculating probabilities" thing? Basically, I'm guessing there is some kind of in-built "out" for Whedon to use to explain why Wanda just doesn't solve everything with a wave of her hands.
But, with someone like Captain Marvel, she's pretty much the ultimate hero, so trying to figure out challenges or even threats for her is more difficult. I'm not sure of the comic book stats, but she's basically superior to Thor since she can do anything he can, PLUS she has energy absorption and projection abilities as well. The energy projection abilities also match one of Iron Man's main abilities. She has strength that excels Captain America. And I'd imagine she could probably challenge the Hulk, even if the Hulk is stronger in terms of brute strength. So, Whedon may have simply meant that writing for a character like Captain Marvel is that much harder because it's difficult to come up with credible challenges and threats. How do you contain such a powerhouse without making them look stupid or ineffective?
Anyway, whatever Whedon meant, we're getting a Captain Marvel solo movie and more than likely a number of appearances by Captain Marvel in various other films as well!