She wasn't developed much as an NPC, but in some ways that was a good thing. I mean if BioWare had developed her further she'd probably have turned into a pointy eared divine spell caster (I like elves but really, couldn't we have one plain old normal human as a romance option for males!). I found her annoying at first but she did grow on me and I admire her young idealism* and sense of adventure. The biggest drag is that you never get to peer beyond the crusade into how Nalia developed these ideas, and to see what else she believes in. I've also like the idea of having an Amnish noble in the party who is more connected to the nobility than Anomen and Keldorn, whom, while nobles, are rather distant from mainstream nobility. Also I admit to liking Nalia's epilogue in ToB more than possibly any other NPC; she really does go on to great things, and it casts the PC in a warm light as being a formative influence on her. I see SoA and ToB as very educational for Nalia...
I think she wasn't rounded out too much because BioWare decided that Imoen would live... I like Imoen but I do think it would have been a nice plot point to have her die as intended.
*I believe very much, that as misguided as Nalia is at times, the very fact that she tries to do *something* speaks well of the woman she will become. She is willing to strive, take risks, make mistakes, and to be wrong (and to learn from it). In an early 20-something this is an enormous strength.
-Starcrunch