I've done as much of the line as I could and filled out the surveys on each quest, but those seem to lack a survey for the entire line, at least up to where I got.

First off, I think the line gives an interesting background and has some really great ideas in it.
I especially loved racing a goblin through a river, dodging bombs etc

The first bit was easy enough, mostly traveling a lot.. which I personally didn't mind at all.
Catching the bird was pretty interesting too.
Then came the three shrines.

First shrine: couldn't activate it, until I found a trick on this forum
That is: jump up/in, then you may get close enough - the same goes for both other shrines, their range is a bit too short.
After that worked, I mercilessly died from a 4.6k starfire (oops, didn't notice all settings change on PTR, so no enemy castbar.. fixed after that blunder) but it went down pretty easy when another druid came along and decided to help me. Didn't get another chance to try him alone.
One nagging issue on being dead, is that I couldn't find a way back to my corpse as ghost, so I had to use the spiritguide (+repairs and res sickness).

Intermezzo: I'm a feral druid specialising in tanking. My gear is about as good as it'll get without going epic, and it sports enough defense/resilience that the typical raid boss will never or rarely land a crit on me plus a good deal of health and armor. Level 70 opponents tend to simply miss me a lot from defense and dodge.

Second shrine: didn't see any problems there, but I did have a companion on it.
The quest text suggests that there's a time limit, but I don't know how strict it is.

Third shrine: the pain...
At this point three parties converged, for a total of 5 druids (both factions) who helped each other do their fight.
I died each and every time, because I had its attention and simply couldn't heal up against the damage it did, let alone cast spells at it except an odd moonfire.
I think this one really isn't soloable, not sure if it's meant to be. I imagine the shrine guardians will be tweaked some.
As a plus, on death+release I simply materialised at a graveyard near Auchindoun. Neat solution to not being able to ghost back to that island.

I like the riding crop replacement you get as reward here, because flight form does not count as being mounted, so no speed bonuses at all (including paladin's aura). Might be easier to make it count as though you're mounted (especially for that aura), but I love flight form anyway <period>

That's about as far as I got, because I couldn't find a party for heroic Sethekk Halls.
Looks like that was the last step however:
clear heroic Sethekk, summon the ravengod, kill it, head back to get swift flying form and a wicked idol.
From the notes you get at this step (listing each spirit's abilities) I get the impression that this fight will be a lot of fun.

That's where I feel I need to comment.
This line serves to get something that effectively replaces a swift flying mount, because you already need to have the skill.
So, a longish line with a *lot* of time consuming traveling (as much as I didn't mind traveling, it did take time) plus a heroic instance to replace 200g, which at level 70 is pocket change.
Actually, I suspect most druids will already have spent the 200g for the speed boost.

I think it's over the top to require that the druid gets revered with Lower City, clears Sethekk Halls on heroic level and then does this extra (I imagine pretty hard) new boss for his swift flying form.
Putting it bluntly, any idiot can grind 200 gold without ever having seen an instance, which is a far cry from being able to enter and clear heroics.
I think this last part will be a showstopper for casual players, because many will need to pug this (not recommended, and that's putting it mildly).

For that reason, I'd like to suggest these two alternatives:

1. Teach the swift flying form just before the step into heroic Sethekk, then do the heroic for the idol and maybe trinket as well (frankly I think the idol is worth it)
Teaching the form could (in the story of the quest) be done as reward for the good work he's done so far, or maybe so that the druid is more attuned with the three spirits he'll be using in the big fight.

2. Split the idol and swift form up:
First do a regular Sethekk with a tough Anzu for the form
Then for instance have the questgiver discover Anzu wasn't entirely extinguished but only weakened and send you back to do it in heroic for the idol (tougher Anzu logically)

Anyway, my overall view on this line is that it's a great deal of fun, and generally manageable, but sometimes near the end you need to have a companion along. It is however very heavy on traveling across the world and back again.
Aside from some tweaking along the line the only "real" problem I encountered was the heroic bit.