So, what's not casual and solo-friendly about the current holy trinity - healer/tank/damage?
Does the fact that MMOG's are geared around their significant content requiring three players not clue you in that it's not SOLO-friendly? If the entire game is geared around my character being only one third of a necessary unit, then the core design of the game is not solo-friendly. When the major content of that game requires that you play that particular role at a professiona level, then that game is by design not friendly to casuals.
The word is that GW2 has done away with the holy trinity. Every character has skills for each of the new roles - damage, control and support, largely based on what items you put in your hands. If the bad guys are getting out of control, anyone can pitch in. Not enough damage? Again, anyone can ptich in. Having less well-defined class roles, a slew of combinatorial skill interactions, and every character capable for any role, what's not casual friendly?
Since every profession can fill each role, then when you're out and about soloing, if different creatures require different tactics, you're fully capable of changing tactics. You're not bound to one kind of strategy. When you, as the solo adventurer with casual-grade abilities stumble upon an event where several other characters are already involved, you can assess their needs and contribute what is lacking - not just whatever you happen to be limited by class. If they have the bad guys under control, you can just step in and augment their attacks/defenses or lay on some extra damage.
That your abilities can interact with those of other professions makes combat in GW2 more of a multifaceted tactical situation than a linear formula of damage, health and healing defined by class. The professions and skills in GW2 seem to provide for almost limitless explorations of how different professions can interact. and offer on-the-fly improvisation.
Is that good for casuals? I think it's good for everyone who has gotten bored of the old, formulaic holy trinity battle system, and it certainly makes it easier for a casual player to be accepted into groups if they have a willingness to fulfill whatever role required and improvise when necessary for the good of the group.