Why is Baneslayer Angel so expensive???? I really don't get it?

Currently, a lot of the standard decks are running Jund type creatures, so lots of Vampires, Demons, and things that Baneslayer has protection from. When Baneslayer hits the field, the protection shuts down the attack, the lifelink keeps you from dying, and if you have any backup to go with her, she wins games. That's currently why she is so expensive. Once the new core set rotates though, she will drop like a rock as she was really a bomb in standard and the older stuff has ways of dealing with her quickly, without vampires and demons. I agree she's not worth the price if you're a casual player like I am. Iona should be worth just as much if not more as she can shut down an entire color from being cast. Iona is more expensive, but you can get her out with Rise from the Grave if you're playing black/white. Use Rotting Rats to discard her and then Rise from the grave to get her cheap. Also, Elvish Piper costs a lot for a 1/1 elf, but if she survives one turn, you can also have an Iona on the field for the cost of 1 green mana for the white/green player. Emeria, the Sky Ruin will also bring her out in late game, provided you have enough plains in play. So I don't understand why Baneslayer is $40 and Iona is only $7.

It is overpriced, but it is also an obscenely good creature. Probably the best mono-color creature ever.

At 5 toughness, it is practically immune to direct-damage spells.

At 5 power with First Strike, it can kill most individual creatures in combat without taking a scratch.

With Flying, it can block almost anything.

With Lifelink, it effectively negates the attack damage of one or more creatures it doesn't attack, so you can leave some unblocked.

All of this for 5 mana of one color. Normally, that would only get you something like Flying in a 5/5 creature. First Strike and Lifelink with it would normally be more like 7 mana in a monocolor, or require 3 colors. The Protection from Demons and Dragons is pretty much garbage, but who cares with everything else it has going for it? Just compare this to Battlegrace Angels, which have the same mana cost. Which is the better card in virtually every deck?

Not sure what you are seeing as "profound weakneses". Sure, Shroud or Protection from Black would be better than the protection it has, but by that definition, 99.9% of all creatures in Magic have "profound weaknesses".

because baneslayer angel is "Mythic Rare"