ExploderWizard
Hero
From a rules stand point. Once you start breaking the level cap, messing with action economy, significantly screwing with damage output, etc, from an item that potentially anyone could get at any time in their adventuring career, it messes with how the rules work with each other.
I'm the last person to worry about balance (a four letter word, imo), but the rules work so nicely the way they crafted them. Why mess that up with poorly designed magic items? The items we've seen so far, don't do that. I'm wondering why people think there will be such items.
A magic item that does basically nothing because of rules balance is more worthless than Awec Bawdwin! 4E taught us that lesson in spades. I don't even remember what magic doodads my fighter in our 4E game has. They aren't worth remembering or caring about but they are very balanced and don't break anything.
I welcome the return of "game breaking" magic items that cause player's hearts to pound just thinking about the possibility of finding them. Its the player entitlement to such items that needs to get chucked out the window not the items themselves. So long as access to these items remains with the DM, and the consequences of placing such items is communicated, I say let them in. I remember the joy finding a vorpal sword or an arrow of slaying in 1E. Those items were exciting to discover because they were so powerful.