I hate Melissan.
Her place in the story makes absolutely no sense, she's as transparent as glass, and her voice grates on me something fierce. And the last battles are a cheese-fest on her part. Only Balthazar was worse in that regard (and I haven't even tried his "Improved" version).
Luckily, Melissan is not too hard for a well-equipped high-level party, if you use the Wish-to-rest trick at least, or if you are careful not to burn your spells and abilities early on. I still maintain that the Final Guardians in WK make for the toughest battle in the game.
IMO, Melissan should never have been the final challenge. Come on, the "idiot mortal with an ego problem wants to steal a god's power" is getting old. I'd prefer her to be faithful to Bhaal instead, trying to subdue the PC as a vessel for Bhaal's return or something like that.
Either way, I wanted
him to play a bigger role in the story (hell,
any role would have been nice - at least he talked in your dreams in BG1), and would like to see some dialog-based confrontation with him in the end (NEVER an actual battle, though.). Hey, I hear Torment did it, why not BG?

"Beware of the blindness of those who would follow, and the damnable lure of those who would lead."
- Tamoko
"Mmm? What's this? You gots hammer? Bhaal once drop hammer on big godly toe. Jump around and swear for days, he did. Kicked poor me all the way to Baator. Very bad week, that."
- Cespenar the imp