There’s a whole list of websites that are trying to compile the working addons for the WoW Classic beta. The easiest one to parse through is Will It Classic. The site has a list of addons — including a search function — and has color-coded each one to signify whether or not it’s working on the beta client. Keep in mind that the client runs on the Legion API, so there are a few semi-current addons that work without any tweaking by their creators.
Fifteen years of iterative design means that the World of Warcraft that existed in 2006 is dissimilar to World of Warcraft: Battle for Azeroth as it exists in 2019, even if you’re playing the same content. The cumulative effect of thousands of tweaks means that pinpointing exactly what made Classic so special is a nebulous undertaking. Flying mounts, for instance, were a welcome addition when they were introduced in The Burning Crusade. It eventually became clear to players, however, that the convenience and fantasy they offered came along with a number of significant drawbacks like reduced interaction with topography, enemy NPCs, and other players. In spite of this, flying limitations are met generally with negative feedback on the part of BFA’s community. Such conveniences are difficult to unlearn.
Brewmaster: Hot Trub - Purifying Brew deals 30% of your purified staggered damage to all enemies within 10 yards. Mistweaver: Way of the Crane - Increases your Physical damage by 25%, you remove and become immune to all snare and root effects and you heal up to 3 nearest allies for 150% of all damage done. Lasts 15 sec. Windwalker: Reverse Harm - Heals a friendly target for 8% of their maximum health, and causes 100% of the amount healed to instantly be dealt to the nearest enemy as Nature damage within 5 yards. Paladin
So committed, in fact, that modern WoW players are trying the beta and reporting what seem like bugs today but what were actually intended functionality 13 years ago. This became such a common occurrence that Blizzard publicly posted a list of known non-issues called the "WoW Classic 'Not a Bug' List." For example, hitboxes for the Tauren player race are much larger than those of other races. In a modern game, this would be seen as a serious balance issue (see: Apex Legends). But it's what vanilla WoW was like, so it has been faithfully reproduced.
When it comes to levelling the Priest Class from 1-60 fast, XP grinding locations is only half the battle. You will need to make sure your weapon does enough DPS so that you can clear these mobs fast enough, it should also be noted that monster should be around your level to make sure to gain the 100% XP Reward. The Level your character should be before you start that specific location will be indicated to left in a Blue Color for Alliance [Lvl 35-39] and the Orange Color for Horde [Lvl 35-39]. The Mobs you are looking for in that specific area names are Bolded [Mob Name].
It is also worth noting that World of Warcraft Classic isn't exactly an archive from 2006. It has been rebuilt from patch 1.12, using modern data architecture and reaps the benefits of the modern engine. This means a lot of non-controversial quality-of-life features will still be present and it is possible to run WoW Classic servers on modern server architecture. As you can imagine, much has changed in the last 13 years in the tech space. That's before smartphones were even a thing. The modern engine no longer allows certain behaviours that could be used to get out of bounds, also adds widescreen monitor support and accessibility options! However, it does have the disadvantage of not being able to run on 32-bit systems which isn't that kind to older systems considering the game is from 2006.