The priest was considered the best overall healer in vanilla WoW. Priests had the biggest selection of healing and defensive spells which could be used in various different situations. Due to this fact, Priests had an easy time finding groups and guilds. If you’ll embark on leveling priest, expect some random whispers from other players inviting you to join their group as a healer. Shadow Priests were also considered as one of the best duelists in the game boasting with serious damage and healing.
Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.
Preface - I haven't played WoW for any length of time since quitting at the end of WotLK. I dabbled in the next few expansions, where it seemed gold was overabundant, not sure if that's what killed the gold farmers/gold bots/sellers, but I didn't seem to see any, or even think I might have a use for them. However, with WoW Classic, I can see a market for them to come back. Did Blizz ever put something in the game to eliminate this after WotLK, or was it the gold abundance that destroyed their market. If not, I see the gold sellers and websites coming back.
One thing they told me when I dabbled into computer science back at university was that 80% of the bugs only take 20% of your time to fix. That's why sometimes you see issues persisting for very long times, not because the devs are lazy or incompetent, but because there is usually a priority list of problems, with the severe-but-easy-to-fix ones always taking precedent and the not-game-breaking-but-difficult-to-solve problems being last line.
And for talent builds. I prefer to make my own talent builds. But I've been playing RPGs since the 8bit days so half the fun is building my own characters up. I already made and bookmarked 1 deep fire mage build, 2 elemental mage builds, and 2 deep frost mage builds. And I might remake them tomorrow if I read up on any good talent choices I left out my 1st 5 times lol. And I have multiple builds for the other classes I play too.