When it was first announced that Blizzard intended to release some sort of remakes of ZG and ZA, people seemed to have mixed feelings about it. Although most people seemed to welcome the opportunity to run through these awesome raid instances again (albeit this time as regular instances) some people seemed annoyed over the fact that Blizzard kept reusing old material, rather than coming up with new things. One question asked to WowInsiders The Queue was "why are people happy about the fact that they get to play old content all over again?" (paraphrased). I am one of those people who is overjoyed about getting to play these instances again, so I'd like to answer that question.
It's not really just "same old all over again"
Alot of people who are negative to Blizzard doing this, treat the remakes like if they were a newer version of the old one. First of all, very little about these remakes is like the old content. In fact, much like the SFK and DM remakes, the only thing that really is exactly the same is the surroundings. The encounters will be completely different. I can imagine the bosses will be true to their theme, ie panther boss about panthers, tiger boss about tigers and so on, but overall the instance will probably be nothing like the old thing except for the interior design.
It's about nostalgia
After a certain amount of time, people get nostalgic about old content, and want to relive the "good old days". Some do it by revisiting old content "manually", running through Karazhan just to get to relive that feeling that you had back then. But there is really nothing like getting a new way to experience old things. This has been used in games (and everywhere else) a thousand times before and it doesn't have to be a bad thing if done the right way. For example, basically everything in a game like Super Smash Bros is reused content. The music are remixed versions of old game tunes, the characters are all from old games and the arenas are versions of different levels from old games. The game would've been so much more boring if they hadn't implemented these nostalgic elements. I think in any game series fans would be overjoyed if the designers decided to implement something from the original game, or some other "old content" into a newer edition of the game. People overall enjoy to be able to replay things that they've enjoyed. Imagine if the newest Sonic game had a level which was exactly like the first zone of the first Sonic? Or if the newest Mega Man had a level which was exactly like Elec Mans? And so on. This kind of reusage of old content is constantly made, because most people enjoy them. You just have to make it at the right time. Reusing content from one game to the next might not be too popular, but these instances haven't been played since Vanilla and BC respectively. Plenty of time has passed since they were current content. Enough time to make them interesting again in my opinion.
Finding a new audience
An even bigger reason game developers decide to release old content all over again is to give a new audience the chance to experience it (aka earn more money). My list of awesome Snes games that I wish were re-released in a new version is long, and that's just from one gaming console. I have an equally long list from just about any old gaming console (FF7 remake probably highest on any of those list though). Same thing with old computer games of course. I have no idea how many Wrath babies there are out there, but plenty enough to warrant reusing old content just to give these people a chance to see these instances for themselves. Like I said, Blizzard are actually re-inventing the old content much enough to have them work nothing like the old raid instances did, but the feeling will still be there (I hope). Re-releasing content is tricky. You want to make it seem interesting and up-to-date for audiences today, without removing too much of the original feeling. This can be done in good ways like with Star Trek the Movie, or in bad ways, like with Tron Legacy. Good: Final Fantasy 1-3 remakes, Resident Evil remakes. Bad: Well I can't think of any bad game remakes right now actually...
They don't have to be mutually exclusive
It doesn't necessarily have to be that releasing old content in a hotter version means Blizzard aren't spending any time actually inventing new stuff for the game. I have no idea of course, but being the optimist that I am I believe they are spending exactly as much time developing new content as they would have in any case, and these old content remakes are just added flavor to the game, a way for them to give us something new to do without having to spend alot of time with it.
Like I said, I am really glad that they are giving me the chance to enjoy these instances again in a new and modern version. I have occasionally gone back there just to check it out, and Zul Aman is one of my favorite raid instances of all. I'm not sure if I'd like it as much if it really was just a revamp of the old content, ie the exactly same bosses just tuned for level 85 players instead. Come to think of it, I'd probably like that too. I never had a chance to raid ZG much when it was current, so for me that would be something new. And the boss fights of ZA were greatly designed in my opinion (Blizzard were kinda in their prime whe they designed the BC raid instances imo, or maybe I just enjoyed the game the most then for other reasons). I am definitely looking forward for these instances, and the only sad thing about them coming, is that the old versions will be gone forever.