On court, his biggest change has probably been tactical. He is now the ultimate percentage player - not something that could have been said about him 10 years ago. He's stronger but slower. More hard-working but has less flair. Yes, he'll rip the ball when needed, but more times than not, he'll make sure it lands well inside the line. Defensive to some extent, but to categorize him as such would be misleading. I think this hurts him against the top players. He played much more aggressively against Federer at the US Open and the results were clearly better.
But don't be fooled. For all the commentators say about Agassi being a completely changed man from his younger days, the same guy still lurks in there somewhere. You can see it on certain days.