The Morninglord paragon path is one of the most ridiculously potent options available to players looking for an offensive boost. The reason is its 16th level feature, Burning Radiance, which reads as follows:
Burning Radiance (16th level): Whenever you hit with a power that has the radiant keyword, the target gains vulnerability 10 to radiant damage until the end of your next turn.
That is a LOT of extra damage, even when used fairly. The obvious (and probably intended) application is a radiant attack from a divine class, with Invokers and ranged Clerics getting the most out of it, due to the abundance of area attacks. However, any weapon-based character can benefit from it with the help of a Sunblade or Radiant Weapon, opening the door for Twin Strikes, Brutal Barrages and other multiattacks, with really impressive (and dangerous) results. Even worse is the use of group optimization to exploit the fact that the radiant vulnerability affects all attacks, not just those from the Morninglord - just stack characters with radiant attacks in your party, and see your foes disintegrate in an explosion of light.
There are quite a few things wrong with Burning Radiance. The vulnerability provided is excessive, it is too easy to apply it outside of the relatively safe divine classes, and boosting the whole party is downright crazy. Also, the rest of the path has a decent power level (with strong encounter and utility powers), so there is not a huge risk of rendering it unplayable by toning down its key feature. With that in mind, this is the version I’d use in my campaign.
Burning Radiance v.2 (16th level): Whenever you hit with a divine radiant power, the target gains vulnerable 5 radiant against your divine powers until the end of your next turn.
This cuts the vulnerability in half, limits it to divine attacks, and prevents it from boosting your allies. It is a huge drop in effectiveness, but I find that the revised feature (and the path as a whole, which still has an impressive area encounter attack) is still a very nice choice for most divine characters. It should no longer be a valid option for random characters looking for extra damage, but that was part of the point.
You can find an errata thread in the official forums for this paragon path here.
EDIT (06/04/11): Changed feature benefit from "divine attack powers" to "divine powers"