The problem with this one is that the wizard can drop it onto a melee, and the monsters eat the initial damage, the start their turn damage, and some AoO damage.
There is no start of turn damage. Unlike many other wizard dailies, Fountain of Flame deals its damage at the end of turn, giving enemies a chance to avoid it. Granted, it can still be comboed with a fighter or cavalier to punish enemies even if they flee, but I'd say it's a significant drawback.
It's a really cool power, and I really like it, but I don't think it matches the power of stuff like Flaming Sphere and other similar powers, which have movable areas and deal automatic damage each turn while getting to add all the bonuses to the damage roll, too. Its main advantage is the fact that it's party-friendly, which is uncommon for wizard spells, but that isn't enough to compensate for the difference in raw power, in my opinion.
I don't think that Stirring Shout is broken, either - I've seen it in action several times, including a few solo encounters, and found that it was very useful but it didn't trivialize the fight like some of the cleric dailies that have been mentioned. And outside of solo fights, it became even less of a problem.
I'll add a few broken powers of my own:
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Come and Get It (Fighter, lv7). This one is broken out of sheer synergy with the features of its class. CaGI might be fair in the hands of a paladin, or just about any melee striker, but give it to a fighter and it defines an encounter like few other powers. It positions enemies exactly where you want with no attack roll, lets the fighter mark most of the opposing team at once, and combines absurdly well with allied area attacks.
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Dishearten (Psion, lv1) and Mind Thrust (Psion, lv1). These are not as problematic when fighting and encounter as when building a psion, but they are still pretty broken in my book. They interact poorly with the power point progression, becoming easily spammable at higher levels and making it hard to justify taking any other power (regardless of level) instead of them. So several dozens of higher level psion at-wills become obsoleted because of them. And dishing out absurd penalties to attack or defenses every turn for a whole encounter, at higher levels, is way too strong to my taste.
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Lightning Rush (Battlemind, lv 7). This one is extremely versatile and potent, and also becomes way too effective at higher tiers where its augment cost is trivialized.
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Brilliant Recovery (Battlemind, lv 27). An unusual psionic attack, in that it's broken while unaugmented. As a minor action attack that is not restricted to 1/turn, it allows for some silly damage numbers if you are able to trigger it, to the point that it might be worthwhile to try to miss on purpose. On the other hand, it combines so well with Brutal Barrage (Battlemind, lv 13) that this is rarely necessary.