Get a life guys. Yrel is perfectly fine. If u want true retri paladin then go play WoW.
First, she is better than Maraad, because she's still alive.
Second, she is probably the only significant female draenei. If you replace her with Maraad, you won't get female draenei in future (or u will get random noname character). And you still have targets for male draenei - Akama or Velen.
Third, there is 20 warriors for now and only 2 are female (Sonya and D.Va).
About her kit, it seems fine but her ultimates looks a little underwhelming. Ardent defender is just a little better Medivh's barrier with 10 times longer cd. Sacred Ground is somehow mix between Tyrael's and Chromie's ults. It's probably good in defending objectives and sometimes in tfs. Seraphim is just Johanna' s trait (half of the cd and duration), it helps against heavy disruption team, but comes too late. Until 20 you would be raped hard. Bubble Hearth should be called Righteous Retreat.