Back in the day Diablo multiplayer was firmly an Everyone is Dps game. This was principally due to the lack of a decent heal. The Paladin class had a weak heal but it was unreliably and hugely over-shadowed by other methods of healing such as life leech and potions.
It's also true that the community was less aware of trinity possibilities. The internet only really became widespread for gaming in about 1996 so in 1999 when Diablo 2 came out most players had just moved from single player offline games where of course dps is the default playstyle (perhaps with self-heals).
I think Diablo 3 will see a lot of trinity gameplay. It's possible to make a decent Barbarian tank based on Threatening Shout which with the Demoralise rune becomes a spammable aoe taunt. For example this one has very high armour, stuns, damage reduction, resist bonus and self-healing.
It's certainly possible to make a decent Monk healer. This one relies on high spirit generation to supplement a formidable range of heals with heal on demand Cyclone Strikes.
What's more the other classes have methods of becoming more group friendly. I'll explore in more depth in later posts but to start with the Wizard and Demon Hunter can crowd control and the Witch Doctor can not only crowd control but adds that nice wall of meat to hide behind.
Regarding treasure generation there are two elements to the equation - kills per minute and magic find (or gold find) bonuses. Being in a group may allow you to sub in some magic find gear over other options because other people have got your back.
Regarding difficulty, well, if a monster is challenging to a soloer, is it really challenging to a very high armour Barbarian who is getting healed? Perhaps not. And the challenge can be reduced by burning the monster down fast.
We've been warned Inferno level D3 will be exceptionally difficult. The devs have reported that they're dying a lot in internal play tests. Perhaps one way to handle that challenge is with the Trinity.
A game designer friend is quite convinced that the Holy Trinity is an exploit that became a feature, that originally games were not designed to cope with a team that presents the best armour, the highest dps, and the maximum healing at the point of contact.