I have a player who wants to play a barbarian similar to the 1st Edition, magic-hating version.  I was thinking of throwing this ability onto whatever sub-class they pick at third level.  My intent is for it to be a "wash" in terms of benefits/detriments.  I was thinking something along these lines:

Magic Averse

Your tribe is averse to and has a superstitious fear of arcane magic.  You have advantage on saving throws against spells and other magical effects.  However, you may not attune to any magic items.

Seeing through illusions and advantage on saves for will related spells charm person etc

That ability is way overpowered. Maybe +1 on saving throws against magic?

Not being able to attune to any magic items is pretty big depending on the level of magic items available in the campaign you're running. If they rarely find any then its not a big deal, if the players are tripping over them and buying them from magic shops this is a pretty potent downside. 

Is it?  Yuan-Ti get it as a racial ability, without the significant downside of no magic attunement.

Yes, but it's a racial ability, which are usually very powerful when used properly. Ex: Dragonborn breath weapon, Wood Elf Mask of the Wild, Variant Human free feat.