Attendance - we are struggling at having two Ulduar groups (or at least we were before our newest raiding schedule), which have the added attraction factor of phat lewts and actually being "endgame". What makes us thing we'll be able to gather nearly 40 people for Molten Core or the like?
I'm not suggesting we run before we can crawl; 20-mans would be the first aim. And something like this is somewhat less hardcore, less demanding, than Ulduar and the like. It'd be open to people in guild and out of guild, and there's plenty of interest. I'm not saying we'd be instantly ramping up to 40 awesome level 60s pimped out in the finest Vanilla has to offer. But I do think we can at least have some fun trying.
That's the point of this entire venture; to try, and to have fun.
It's not Vanilla - the classes as they are even at 60 vastly different from what they were in Vanilla at 60. The encounters are designed and tuned for them as they WERE, not as they are now. In this sense, the "nostalgia" factor will be hardly fully fulfilled, as the classes will be just very different, as will be some of the gear.
I know they are different. I've been here since 1.6 or so, I've seen the classes grow and change with each patch and expansion, and I've run some level 60-ish 5-mans with a group of 3 people and not had that much trouble. I'm fully aware that we're stronger and the instances are no longer tuned quite to us. I know we can never get quite the same experience as before.
But this is exactly why there'd be certain restrictions and measures; the not allowing gear, enchants or consumables from Outland or above, the not allowing unglyphed knockback abilities, the restricting of the number of DKs and banning some of their abilities (and possibly abilities of other classes, depending). We can never get it to exactly how Vanilla was, but we can get close, and we can try and make it a challenge, try to keep it fun.
Also, to address the fact that even with these restrictions we'd be inherently stronger than we would've been back then in the same scenario, we'd try taking fewer people than was intended for any given raid to try and balance it out; 15 people to ZG, 30 to MC, for example. That could be increased of decreased to try and find a sweet spot, and would also cut down on the numbers issues from your first point.
Again, for emphasis; we'll never get back to Vanilla with these efforts. But we can get close, we can try and have fun with it, and that's what really counts, right?
Synergies - I'm not quite sure if this could be a problem, but with only 51 talent points we will be weird. Some essential synergies will have to go in vain, because we won't have enough tp to use them. The talent trees are mostly designed to make sense for lvl80 characters and might fall apart when tested heavily on lower levels. We will also miss some essential spells/abilities (I'm thinking Divine Plea for tankadins, Shadowfiend, Prayer of Mending, stuff like that that makes the class click in its current state).
I know some abilities will be missing, and I know that we'll be restricted on talent points. But that in itself is a challenge, isn't it? Will you spec only for one tree to get your 51 point talent? Will you sacrifice that final talent in order to spec into two trees? I know not everyone is as interested in theorycrafting as me, but I do think that people can have some fun playing around with these restrictions to see what they can come up with, and it may well make for some more unique and individualistic arrays of talents than we normally get to see.
Also, I know that the lack of certain abilities may severely hinder some classes, and if it's the case that it's a critical flaw for a given class/spec to lack certain abilities while we try this raiding, we could always try to compensate a bit by lifting a couple of restrictions (allowing certain otherwise-banned consumables, gear or enchants to help alleviate the problems).
Now I know some of these (well, the latter two) points are not rock-solid and may very well not be an issue, but I got slightly concerned about them and thought I'd share them with you

Because other than them (and mostly the attendance thingy) I'd very much like to enjoy some Old World action!
They're all very valid concerns, and ones that occurred to me as well. I'm certain that there are ways to deal with most of the possible problems in one way or another, and at the end of the day, the ultimate objective is for us to have fun, not to have a 100% accurate vanilla nostalgia experience; we'll make adjustments to keep it all fun!
Finally, to address a query that's been sent my way by various people:
Yes, I am still planning on doing all of this, even if I've been very quiet about it. The reason I've been seemingly doing nothing about it is that with the release of 3.2, I know that many people are eager to play on their main characters, adapt to the changes, gather Conquest emblems for level 80 gear or more Heirloom items for their alts, etc. I've been waiting for everything to die down a bit before launching anything major.
At the moment, we're looking pretty reasonable on numbers, with the only thing we might be lacking being tanks (in a 40-man context, at least; we might be okay for 20-mans), but that's not too harsh of an issue at present. Many people are levelling up quite rapidly and will likely be ready before too long. Once we have enough people in the level range where they can run, say, Strat/Scholo/DM, then that'll likely be the point at which everything can be put together more seriously.