Idraen

I'm slightly perplexed by Idrean. As things stand she seems a relatively weak hero, with Warden of Arnor attached she is slightly better but hardly overwhelming. Unexpected courage seems a much more versatile and reliable way to ready a hero. The inevitable comparison is Glorfindel + Asfaloth + Light of Valinor. As things stand she is just not good enough to replace this powerhouse, but hopefully the Scout / Dunedian trait will get some more support in the future. 

She (usually) allows you to quest knowing that you will have her standing once the quest has completed. You could even use Late Adventurer to add her to the quest once you know you will completly explore a location. She can make use of Desperate Alliance to let another player use her for attacks after she has quested. She's great for Strength of Will because she will stand when you explore the location and then she can exhaust to add two tokens to the next one. If it's a small location, she might stand again! She teams up well with Northern Trackers and Legolas because they will both stand her up in other phases that aren't the quest phase. Give her a Gondolin Blade or two to let her stand herself up after an attack that kills. It makes her a good option for spirit to be part of battle quests with her and Glorfindel.

She's got a lot of potential, but I agree, she hasn't hit her stride yet. I suspect the box featuring the Dunedain will boost her significantly.

You're right, Strength of Will, specially with a 2nd small location, is a great event to use with her. Explore 2 locations in the same round and still having her prepared to combat sounds pretty good.

This would have made a great threat 9 hero, but 11 is laughable.

She has a built in readying effect and 3 attack power in spirit.  I feel that 11 is justified.  Of coarse she would be much better with the same stat line and only 9 threat, but that would fall outside of FFG's standard stat line = threat practice.  

^This.

Sure, her threat is certainly high. But she's in Spirit. If you're really concerned about it pair her with other low-threat Heroes (of which Spirit has a multitude of solid options). Or pack the deck with threat-reduction--also Spirit's thing. 

And don't forget that she makes a great splash in a combat-focused Tactics deck to give them easy-access to readying, cancellation and threat-reduction (not to mention the 2 willpower) without taking too much of a hit in combat-efficiency. 

Honestly she's a pretty decent Hero. I think the reason you don't see her played as often is that Spirit has a lot of really good Heroes. And Eowyn+Herugrim+Unexpected Courage is probably the better option unless your building a deck specifically to her strengths (Dunedain, Ranger or Location Control). 

She's like the Prince Imrahil of Spirit except with a repeatable standing effect and excellent traits - Ranger in particular.

She makes an excellent pair with Spirit Glorfindel. The latter with a LoV attached and Idraen with her built-in standing effect guarantee action advantage in the form of both strong questing + attack power. And let's not forget that Glorf's omnipotent steed contributes tons to readying Idraen. And Glorf's low threat mitigates her high one, of course.