Analogy: think back on riding the swings in kindergarten. Was it fun? Now, imagine going back to those swings exactly as they were, and sitting in them now, as an adult. They're too small. They don't fit. Your feet drag on the ground because they're so low to the ground. The bar over your head is low enough that you can reach out and touch it, and even at the highest the swing will go, it's only about chest high when you're standing up. It's exactly the same swing, but riding on it now is a very different experience, yes?
Why was this downvoted? The swing analogy is horrible. There’s nothing physical that’s going to change when u play the game the exact same way as 04 05 06. The difference is u know what’s going on as opposed to not knowing where anything is. A good analogy would be playing Pokémon red again after beating the elite 4. The game itself won’t be any worse, it just will be easier and maybe less enjoyable if all you enjoyed is the aspect of not knowing where to go. But this is why pvp servers are magical, because each time you level a character, your leveling experience is different based on the encounters that happen while questing. In 05 U may have breezed through stv, but in classic u may spend an hour on a single quest solely because of the competition between factions on that particular quest.
To actually get access to the beta, which has already started and continue to add more players, you need to sign up via your Blizzard account management page. Under Games & Subscriptions, scroll down to Beta Access and visit the Beta Profile Settings page. Once there, you’ll see a grid of available betas you can opt into. Once you’ve checked WoW Classic, hit Update Preferences and you’re set.