The biggest change that I remember was done to the bomb throwing giant in Sen's Fortress. In 1.00, he used to predict where you're going before throwing the bomb and it would really make the first few attempts to go through be extremely annoying and difficult. I loved it.
I've got a 1.0 version ready to go for the next time I feel the itch to play Dark Souls, but haven't given it a try yet. I've even gone so far as to build a character for when I decide to.
A few other changes - infused weapons like fire and lightning did much more damage, famously, the Ring Shield's attack was based on your weapon in the other hand, and could do BONKERS damage, DWGR activated at 50% load instead of 25, so flipping havel moms was a thing, and aggro range for most enemies was toned waaay down. Also, boss drops that are automatically put in your inventory like the Leo Ring were actual drops that were missable. It was a crazy time.
Back when your weapon scaling didn't determine who you were capable of invading, the Undead Burg was full of invaders that beat the game at level 10 with maxed-out weapons.