Just a few questions from a new player!

I was looking at joining either Onyxia or Icecrown, as those were the highest population servers.

I'm just wondering what the main differences between the two are. I see that Onyxia recently launched and is 3x compared to 7x Icecrown.

The next question would be regarding classes, I only played a small amount of time on Retail, but was curious what sort of major changes there would be to the classes/how they play.

I'm not sure which class I would pick either, to be honest, but was just curious if there was anything that I should know going in.

Lastly, is there any official discord that might be a better place to ask questions in live time?

Appreciate anyone who can help me out!

Onyxia is 3.3.5 client using Vanilla-TBC-WotLK progression realm. It's in some early vanilla phase still. At the end of Onyxia, characters can be transferred to Icecrown.
Icecrown is in permanent 3.3.5

If you download the 3.3.5 client from https://www.warmane.com/download, you can test both realms. If you're trying and figuring out WoW, I recommend starting with Icecrown honestly. Progressive realm might be fun, but it changes every now and then, so class you like in Vanilla phases may not be something you like in TBC or WotLK, or vice versa.