When Apple released iOS 7 they included some tweaks to the Mail app including a way to customize message flags to suit your tastes. If you aren't a fan of the default flags that appear, there's now another option. Here's what it looks like and how to change it:
I've never been a huge fan of the flags and was glad when Apple finally provided the option to choose something different. I much prefer the standard shapes. Let me know in the comments which you prefer!