Sun Shield, the capstone ability of the Sun Soul monk, essentially allows you to burn your reaction to deal a guaranteed 10 points of damage against someone who hits you.  This is about what you'd do on average burning your reaction to make an opportunity attack.  I mean, it's a guaranteed "hit," but it's still situational and seems awfully weak compared to quivering palm, etc.  Or am I missing something?

It seems like it at least shouldn't use your reaction.

Yeah, it is a little underpowered. Few subclasses are as good as open hand.

Sun shield is basically a bonus action light cantrip with built in hellish rebuke. It is a little weak, but it doesn't cost any resources to do.

Getting hit is far more common than enemies triggering opportunity attacks, at least in my experience, so you should be able to make use of your reaction more often. 

If you were to decide to make it not cost a reaction, it would have to be limited to once per turn some other way or it would become overpowered.

Quote from Lunali >>

Getting hit is far more common than enemies triggering opportunity attacks, at least in my experience, so you should be able to make use of your reaction more often. 

If you were to decide to make it not cost a reaction, it would have to be limited to once per turn some other way or it would become overpowered.

 Definitely.