first of all, there is the co-op game play and solo game play - secondly 1 of the best things bliz said we will do is situational build which means through 1 act u might make about 2-3 builds according the mobs u need to pass and the envirment( my speculations , but not sure), as for i didn't play the game yet i can't know for sure which build i want to aim at. my choice of builds will be according to my experince in diablo2(which might not be relievnt cuz d3 is considered diffrent from d2). ofcourse i will adjust my build according to the needs of the party i play with.
so my current build will have some skills that help the grp but it's mostly for solo play style. as much as 6 buttons r not much i don't want to spam all of them all the time so i will look for some long duratiuon spells with passive buffs/dmg.
secondly i think a pet is always useful to split the dmg or even "tank" for u abit so i will try always to have some kind of ally spell or familiar.
ofc i will try to have some kind of heal rathier it will be "life steal", side affect heal or direct heal. if we r talking about resource genertaing skills like the monk barb and DH,
i will suggest a generter skill with the obvois spender skill and not count on items that gives resource regan or some kind of side affect of the skill spender- as a main resource regan skill.
and an obvious dmg dealer spell in 1 way or the other - most of the time u will fight a grp of mobs, so i suggest a first priorty is an aoe dmg dealer and second 1 as single target massive dmg (for bosses and stuff) - and also have atleast 2 different kind of dmg(for example: fire and lighting dmg...and so on...), for mobs which might have resistance for certain type of dmg(i understand there won't be mobs with immunity to some type of dmg like in d2, but still )
lastly according to the kind of class/build u want to have a utility skill like knockback skill for ranged or dmg mitigation/def skill for melee.
as for the second question u asked - firstly i will try all the skills and see how i feel about them , tho u need to remember that runes might not drop so often so u might get limited skill tests for a while, other than that i personly prefer to get used to the build i thought of and start with that(most likely i will change the build from my pre release 1 cuz of the simple fact that i don't know how the game will go and if i will enjoy the build).
those r my thoughts about my choice of build - hopefully i mange to help u abit, nevertheless u should do what u feel the most suited build that u enjoy the most,
P.S: sry for all the missspelling and grammer - english isn't my native language