Can someone please explain melds and stat weights to me?

[Vidu]

Are there any jobs that don't get as much benefit from crit? DPS-wise?

As Paladin I take DH over crit, because of how it scales (as Moro explained) and not having any attacks that benefit greatly from crit - its nices when stuff crits, but DH is better (at least it was when I pentamelded my last accessories, I havent checked since). The other two tanks prefer crit or a mix of crit and det though.

I'm not super familiar with the theorycrafting behind BLM, but my boyfriend prefers spellspeed over any other substat - given that a BLM has infinte ressources they probably profit more from casting faster and more than from an increased crit-rate.

I dont know enough about samurai, dragoon, ninja and maschinst to speak to their crit needs. But I would imagine all of them being crit first.
Same goes for RDM and SMN.

Monk and bard benefit extremly from it.

Since crit also effect your healing-spells, its specially scholar that wants a lot of it; with WHM not so much - DPS-wise they both probably enjoy it though. I dont think that my static-whm favours crit, though.

[MoroMurasaki]

Since crit also effect your healing-spells, its specially scholar that wants a lot of it; with WHM not so much - DPS-wise they both probably enjoy it though. I dont think that my static-whm favours crit, though.

Not that I'm a first-rate WHM by any stretch but I stack crit on WHM. There is no specific benefit or crit mechanic to WHM but a nice Assize Crit during Trick Attack is love.

[Vidu]

Not that I'm a first-rate WHM by any stretch but I stack crit on WHM. There is no specific benefit or crit mechanic to WHM but a nice Assize Crit during Trick Attack is love.

Yeah, I didnt mean to say that WHMs shouldnt meld crit - theres no real downside to it (except the occassional crit-overheal, maybe) but also no extreme upside.
So for both WHM and to a degree AST its probably "do whatever you like best".