Before work began on World of Warcraft: Classic, it was only possible for players to experience the original World of Warcraft by using private servers, which are illegal, often have stability or corruption problems, and generally are very imperfect recreations of the authentic World of Warcraft experience. As much as Blizzard has been aware of the desires of their community, until recently it seemed impossible for them to emulate Classic servers due to the technical hurdles of essentially having to run two massively multiplayer online games side-by-side. A breakthrough was then achieved that made it possible to run Classic servers on the modern architecture of current World of Warcraft servers.
Blizz stated in a watercooler that they're using the 1.12 database as a starting point. I don't think it's a stretch for people to assume that most game features and mechanics will be in their 1.12 state. The kind of stuff I would expect to see reverted would be numerous AV changes, but I seriously doubt class mechanics are going to be reverted. I'd be surprised if the threat changes that ruin pre-naxx encounters are reverted.
After this past year of working on this project and forging our way through the various bugs and challenges, one consistent theme that’s emerged is that the difference between what we have and what we want is clearly visible. When we look at today’s World of Warcraft, we can see the differences between the modern game and the classic one. If we tried to update the reference client, we would have instead been tracking down a lot of “invisible” changes such as exploits waiting to be abused, crashes that don’t show up until you have millions of players online at once, and more. We chose to approach the problem in a way that makes our job clear and obvious instead of difficult and hard to see.
For classes who rely on energy regeneration such as Rogues, things have also changed significantly since the classic era. Regeneration used to occur in chunks based on the server “heartbeat.” So, to gain 20 energy, it would take a full two seconds. Today, the system is smoothed out so that that 1 energy regenerates every tenth of a second. This difference changed the timing of being able to use certain abilities. What seems like a minor convenience today actually changes the very flow of combat rotations and burst capabilities in classic gameplay. We felt this was important to restore to provide this authentic experience.
Druid is one of the most versatile classes across all of World of Warcraft Classic. By picking Druid you will have plenty of options to choose from while leveling. However, in the end-game, most groups will expect you to heal, though, Feral and Balance druids are also very desirable for their Innervate and Rebirth abilities. Additionally, various animal forms will make traversing the world into quite an uncomplicated task.