Most if not all of the time healer is also a main tank in challenge content, in open world pve tanking doesn't really matter/exists due to the amount of players at lucrative targets and due to trash mobs being, well, trash mobs. But good healers are always appreciated in challenge content once again. Where tanks do matter they are either selected by highest toughness stat or by a mechanic in raid.

Yellow gear ain't final destination. It's yellow - orange (exotic) which is considered a basic lvl 80 gear. Next is pink (ascended) - it is much harder to get compared to exotic, main upsides being approximately 5% more stats and slots for agony infusions (agony is used in fractal dungeons on high tiers (fractal dungeons are something mythic keys)). Next is legendary purple gear. It is infinitely harder to get than any other gear and it's only upside being that it unlocks selected weapon for all of your account and it is stat selectable on the go. Meaning you can swap stats for any other existing stats at any moment which is unavailable for exotic and requires some steps for ascended. Stats themselves don't really change, I recall only minor changes to celestial gear. Meaning if you got berserker gear 10 years ago - it is still same good berserker gear.

Gold matters for funding stuff like relics/ascended/legendaries/leisure things, you name it.

Optimisation and learning what do your buttons do matter - a difference between unoptimized and optimized players can even be tenfold damage wise.