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.
Oct 24 A Return to World of Warcraft: Player Guide So you’ve decided to return to World of Warcraft. Welcome back! There’s just one thing you’ll need before jumping back into the fray—knowledge. To aid you in making your glorious comeback, we’ve compiled some handy information every returning hero should know. Once armed with knowledge, nothing (and no one) will stand in your way. http://us.battle.net/wow/en/blog/8876440/Nethaera214 Oct 24
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.
As you level, you will buy new ranks of spells. If you splurge out on an item upgrade, you will not have enough gold readily available to upgrade to the latest ranks of spells: placing you behind. Moreover, you can buy gear upgrades from the Auction House (AH) or vendors. Furthermore, you may have large investments that need to be paid for professors. You will need to purchase reagents and items such as leather, bolts of cloth, alchemy supplies – unless you have a charitable friend or another high-level alt.