He's a victim of circumstance as much as Harvey Dent is - he was an arrogant sod for his entire adolescence, but he grew up being the Order's golden boy, chosen one, child of prophecy, etc. His ego was huge, but it was inflated to be that way. The Anakin we see in Episodes I - III and the Clone Wars series is a good person. He cares about those he loves, he jokes around, he takes his job as a Jedi Knight seriously. Episode II shows us his worst side - Padawan Anakin is characterized almost completely by his arrogance and stubborn attitude, and it's hard to like him in this movie because of it. And while he does ultimately carry these traits with him as a Jedi Knight, they're clearly subdued by that point. Another trait that's typical of "good" characters is naivety, which Anakin has in spades. Couple this with his impulsiveness and the Jedi Council's own off-putting attitude towards him, and it's no wonder that Sidious was able to play him like a fiddle.
Anakin was written to be an ultimately good, but hugely flawed character. Yes, his transformation into Vader is the ultimate realization of his flaws, but he was also clearly pushed along that path, and wouldn't have pursued it if not for the cocktail of bad events and shady characters surrounding him.
I don't disagree with what you say about his downfall not being surprising - it isn't - but I also don't think that means that all the blame can be placed on him. You do have a point with it not being as tragic of a story, but was it ever meant to be? I always pictured Vader as someone good who was seduced by, rather than fell or was forced into, the dark side. I never once got the implication we weren't supposed to look down on him for his defection. That said, in the OT it was really all up to interpretation, so I digress.