I've been reviewing some of the major hermetic flaws and I was curious about the forum opinion regarding which of them people find the most interesting or flavorful in concept, even if you wouldn't jump to play a character with the flaw.
I'm personally very interested in Constant Expression (I like the idea of a wizard being constantly surrounded by a tide of magical effects) and Unstructured Caster (I think building a character around such a flaw would be a unique experience).
Vulnerable Magic (HoH:MC p.86), with the "right" conditions can be very flavourful. I like the example in HoH:MC "name spoken thrice": if you are a caster with VM, speaking three times the target's name will dispel your ongoing spells on him, while speaking three times your name will dispel all your ongoing spells. Vaguely Black-Company-ish.
Also, it's very interesting to think about a) how some conditions might be exploited for the caster's benefit and b) some concepts will lead to the caster adopting certain behaviours to ensure that VM does not come into play. Imagine ... VM with the condition "(deeply) loved". Your ongoing spells fail on those who are deeply loved by someone, and if someone falls deeply in love with you, all your ongoing spells fail.
Male character with Mythic Blood: Merovech, had Necessary Condition - (long Hair must fall below shoulder blades).
Apart from being socially unfashionable/morally questionable, the Necessary Condition can be broken so easily - eg a gust of Wind when trying to fly, hanging upside-down, floating in a lake. Never mind swinging blades, fire spells, and a crazed faerie hairdresser (troll who lifts victims up by the hair).