This guy knows what he's talking about. Whenever these Classic servers appear, they are going to be using as much of the modern day WoW client as Blizzard can get away with. There are good reasons for this outside of what Yuyn mentioned. Blizzard isn't just going to toss someone's repack online and call it "Classic". They need the entire under the hood workings down pat so they can provide a good experience on modern hardware while still maintaining the feel of the classic game. Don't expect a 100% faithful recreation of Vanilla... 90% maybe. That's not to say Blizzard won't try to hit 100%, but there are certain things that will have to change.
That Bnet integration will be part of Classic is a given. It's going to happen because all current Blizzard games use it and Classic will be no exception. The Addon API will likely be the same as the current modern client. Any changes Blizzard makes to the BFA client API will likely occur to Classic as well. It creates a uniformity that makes Classic easy to maintain and squash exploits that may crop up. Old vanilla addons will almost certainly be incompatible with Classic but some of the current addons players use now may well work with it since the API is very likely to be the same as the live client. Any old addons that don't have a modern day version will likely need to be remade or updated at the very least.
The Classic client will almost certainly be able to handle the higher resolutions of modern hardware. Blizzard might very well use the higher res textures, lighting and character models of the modern client simply because a) they can be made optional, b) they don't change anything about the classic gameplay experience and c) they look good and lend themselves well to promotional material. Make no mistake, when this goes live, Blizzard is going to milk it for all its worth publicity wise.
While I know no more than anyone else here, I've got a very strong feeling we're going to get a "Vanilla: Remastered!" rather than a 100% identical to the original 1.X days. I'm also fairly confident Blizzard is not going to go through the whole 1.X patch cycle in stages. I expect it will be some iteration of the 1.12 patch with maybe some elements of the older patches (like say pre nerf AV for example) mixed in to tug those rose colored heartstrings. I would not be surprised if Blizzard staggers the content however. Classic can go live as a 1.12 iteration without AQ and Naxx being available right away. But anyone who thinks they are going to have an experience identical or even close to "day one launch" with its crappy talent trees and other issues and then months/later Blizz will roll out all the overhauls, is just dreaming.
Realistically, Blizz is going to make this a "one and done" project that they can use to lure back the old guard and maybe yoke in a few new players to boot. There isn't going to be any ongoing development outside of maintaining the project, fixing any bugs, and perhaps the staggered unlocking of content over time. Eventually it will all be unlocked however and that will be that and everyone who comes in after will have to settle for it as is, just as those who came in at the end of vanilla had to. Only they won't have a TBC to look forward to unless Classic is a huge enough hit that Blizzard decides to make Classic servers for each xpac era (a highly unlikely scenario).