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And Avacyn is a badly designed card. Let's talk about why.

Alright, so one of the main premises of the Innistrad block was that everyone was waiting for Avacyn to come back and save everyone and everything from the monsters. And then Wizards puts out Avacyn, Angel of Hope. It's a huge creature that can't really be effectively played in any straightforward way, and she is probably not going to prevent you from losing in a traditional game.

How did they mess this up? The whole premise of Avacyn as a character is "she shows up when the time is dire and saves you", not "she shows up too late with a flashbang that may doesn't actually help you if you're at 2 life and no other creatures". The flavor and mechanics pretty much wrote themselves down for this card. On top of that, she's the titular character of the whole set, but is actually the most mechanically boring and overall ineffectual of the 5 Legendary creatures introduced (Griselbrand, Bruna, Sigarda, Gisela). You can't really build a deck around Avacyn, and by nature she's not really any more of a worthwhile addition to a relevant deck than the other 50 huge monowhite Angels that exist. Gisela is a better "Avacyn" than Avacyn.

I was like "okay, they whiffed. maybe they just couldn't think of a good and interesting card so they played it safe"

And then RtR comes out and Angel of Serenity is released. Angel of Serenity is a reasonable costed giant beater that immediately swings the game in your favor if she hits the field, and has the definite ability to buy you a good amount of time. Removal be damned, she's already done her work when she came out. This card should've been Avacyn. It's everything she should've been, both flavorfully and mechanically. They most likely had this sitting around by the time AVR was in development, too.

Wizards should obviously fire someone over this

Avacyn's ability is actually awesome and totally flavorful. She's not here to save you, though, just to save her people.

Also, I'm not sure an absurd enters-the-battlefield ability in a mythic would have been the best in a set about blinking. Maybe, but I'm not sure.

It would be cool if you could know important storyline cards are going to be tournament staples, but Avacyn's idea is fine, and I wouldn't have liked that ability being pushed.

There are many other things Wizards should fire someone for. This is not one of them.

Set development has been pretty garbage since Time Spiral block ended. The only good bump was innistrad and DA. Wizards needs to put the bad developers on design teams (or elsewhere), then hire people who understand game theory and competitive play to work on development.

It seems that the OP's problem is that Avacyn isn't very standard relevant. That's not the point. Avacyn was supposed to be powerful and impressive, which she definately is. She's just not as suited to standard as she is to other formats like edh. Since flavourful legends are much more important to edh than they are to standard, that's what they end up getting designed for.
I have to admit though that Angel of Serenity does mtch up with Avacyn's flavour pretty decently. Exiling things is definately what Avacyn does in the story, and the etb ability kind of plays up her escape from the helvault. I wouldn't doubt Serentity started life as an Avacyn design. I kind of suspect that the choice of which one became Avacyn had something to do with how the two cards' abilities interacted with being on a legend.