Patch 4.1 is hitting the PTR Soon™ and in the midst of flurry of blog posts about it, let me be the first to say: ZA & ZG what?
We had gotten hints at the beginning of Cataclysm that ZG/ZA might have merely been a setback with all the chatter about the Tome of Polymorph: Turtle and the vague gesticulations concerning the ZG mounts. The literal revelation that these are coming back though leaves me a bit floored. To a certian extent you expect a bit of redundancy within MMOs, for several reasons. For example, can you imagine 120 different Fire Elemental models? It would be absurd, a huge drain not only on development time but also taxing on players' mental bandwidth. The other reason why you should expect redundancy within an MMO is simply because of player churn. For however X number of players remember going through ZG and ZA back in the day, there are Y players who never did... making the content new to them. If recycling content gives the designers space to make all-new content, that is an acceptable trade-off.
That said, it remains to be seen whether we will be, in fact, getting gameplay returns for developer recycling or if Blizzard is simply repackaging our shampoo into a "new and improved" 9 oz bottle (down from 12 oz) while charging the same $15. Revamped Onyxia was better than no Onyxia in Tier 9 content, unless we could have had Completely New Boss X instead. Blizzard promised Throne of Tides - Abyssal Maw in 4.1 though, so unless that makes it Live in addition to the other two, it is conceviable Blizzard is peddling some Adjustable Rate Mortgages our way.
Ultimately, the above kind of gut reactions miss the PVSAH mark. The only relevant aspect is: are these heroics/boss fights fun? If Blizzard makes them fun, what does it matter if in patch 4.2 we see 5-man Karazhan (hopefully with Chess Event and the legions Malchezaar commands intact) or even the ultimate slap-in-the-face Magister's Terrace revamp? Fun is fun. And while that may seem like a tautological cop-out, at the end of the day it is the only thing that really matters.
Edit: I wrote the above a few days ago when the changes had just been revealed. Since then, my feelings have shifted more towards Acceptance. The Deadmines revamp was essentially an entirely new, innovative instance wrapped in nostalgia packaging. The same could sorta be said about Shadowfang Keep, but I hate the place honestly. I don't like running Deadmines either - both still take even a guild group close to an hour to finish - but if the ZA & ZG heroics turn out like Deadmines from an innovation standpoint, it's win-win for everyone.