I don't like it.
Firstly, I don't agree that facing is a big problem in the case of the Beholder, since it doesn't use shields, can move equally well in all directions, and is immune to flanking. So, while I can see the problems with flanking in the general case, I don't see why they're an issue here.
Secondly, while I generally dislike save-or-die effects, unless and until there's a new edition they are going to be a feature of high level games. And since the Beholder is a CR 13 creature, that places them amongst the high level foes. Therefore, I don't think an obsessive need to remove all such effects from the Beholder is warranted.
I also disagree with the assignment of Charm Monster (and perhaps Fear too) as a save-or-die effect. If the DM and players trust one another, I see no reason not to allow the player to continue to run his character while charmed. True, this will take monitoring by the DM, and perhaps an occasional (very occasional!) over-ruling of the player's choice of action, but I don't see why it shouldn't work. Also, I don't see how you can really run a successful high-level game without trust between DM and players.
However, there are some things I do agree with. The Beholder's anti-magic cone is a real pain. Likewise, the eye stalks do need some rethinking.
In fact, dealing with the anti-magic cone isn't perhaps as bad as is feared. Provided the DM has access to the character sheets, there is no reason he can't put together a set of stats for the character without magic. There are relatively few modifiers that can then apply to the characters at that point (rage, fatigue, a few others), which could be handled on the fly. Still, that requires the DM to do yet more prep work, and isn't a perfect solution in any case. I don't have a better solution, but I don't think Mearls' proposal is the way to go (from a 'it just feels wrong' perspective).
For the eye-stalks, I think the save DC is too low, especially if the Beholder also has to hit. I further think that Inflict Moderate Wounds should be replaced, probably with Inflict Serious or even Inflict Critical Wounds (it's not the flavour of the ability that's wrong - it's the damage done. So, increase it!). Charm Person is redundant if you have Charm Monster. And Finger of Death and Flesh to Stone should be replaced. In at least one case, I would go for Hold Monster, since although it does remove the PC from the game, it also allows a save each round, so isn't an all-or-nothing proposition (well, not quite). I'm not sure about the others - part of me wants to suggest Ray of Enfeeblement, but that's a spell that I've found particularly complex to apply on the fly.