This is probably the closest statement to my conception of how the wish complexity thing works. There's nothing against having a complex, thought out wish--you just have to really mean it. If you're Sayaka, and your heart's desire is to heal Kyousuke, and Kyouko points out to you that, hey, you might as well wish for the power to heal in general...well, the power to heal wasn't really part of your intrinsic desire, but the thought planted in your head makes it hard to wish otherwise...so your potential goes down, or disappears entirely. This is what Kyubey alludes to in Ch. 1 of TtS, when it states that it's better to make the contract now, rather than risk anyone overthinking it.
Plus, you know, the whole limited-time offer effect.
Madoka, on the other hand, probably had enough potential that she could have handed Kyubey an entire page of less grandiose, non-universe-scale wishes after thinking about it for a year and still had the wish work.