Well, technically
Eragon was beating the living carp out of Galbatorix now that he knew the name of the ancient language, so Galbatorix completely obliterated himself (aka magical suicide) using a spell with the power of a nuclear bomb.
However, I do have to agree that
the way Islanzadi was, to best put it, 'removed', was rather pathetic.
Paolini did acknowledge that he left many things unanswered.
Most notably, Angela and backstory
What Menoa tree took from Eragon
What Brom whispered to Eragon for use in times of great need as Brom died.
etc.
Also, the way that Paolini fulfilled the last prophecy was extremely depressing.
Overall, a good book though. However, like you stated, it has so much more potential.
"In many ways, the work of a critic is easy. We risk very little, yet enjoy a position over those who offer up their work and their selves to our judgment. We thrive on negative criticism, which is fun to write and to read. But the bitter truth we critics must face, is that in the grand scheme of things, the average piece of junk is probably more meaningful than our criticism designating it so. But there are times when a critic truly risks something, and that is in the discovery and defense of the new. The world is often unkind to new talent, new creations. The new needs friends. Last night, I experienced something new: an extraordinary meal from a singularly unexpected source. To say that both the meal and its maker have challenged my preconceptions about fine cooking is a gross understatement. They have rocked me to my core. In the past, I have made no secret of my disdain for Chef Gusteau's famous motto, "Anyone can cook." But I realize, only now do I truly understand what he meant. Not everyone can become a great artist; but a great artist can come from anywhere."
-Anton Ego/Peter O'Toole (1932-2013)