I havent played this game for a while. A couple of months ago I did start a mage character and flew through the game up to blight town. Then I remembered how much I hate blight town, and quit. Nearly nothing gave me trouble though until the giant trolls in blight town which my magic was not strong enough.
As for game breaking things, the only thing I know of was the steel skin ability (not sure the correct name). It makes the 4 kings ridiculously easy, which imo is the hardest boss in the game. I struggled with that boss more than Ernstein and Smough. However I had no magic abilities on my first character. The 4 kings had no where to hide while Ernstein and Smough you could figure an attack plan without using some cheap ability.
I HATE blightown too. Everytime I go there I make sure to go in from the Valley of the Drakes or by using the master key from Lower Londo Ruins.
If you come in from the Depths it's just frustrating and annoying! This time around I did that and ended up dying twice on my way to the bottom because of those stupid giants.
Overall the game is much easier for me after nearly 3 playthroughs.
Senn's Fortress was super easy this time since I know my way around it. I also managed to fly through Anor Londo somehow, which is rare.
Ornstein and Smough are a nightmare for me but mostly due to my playstyle I think. I actually cheat on Super Smough and keep him behind a broken pillar. Somehow I got him on the 2nd try.
I have Crystal Caves and Giant's Tomb coming up and despite them not being incredibly hard, it feels like it's going to be a chore.
I hate the invisible floors in the Crystal Caves and those dang Skeleton Dogs that take off half your life in Tomb of Giants.
BTW right now i've found the best weapon for me is the Lightning Gargoyle's Halberd+4. All the other lightning weapons do crappy damage but this one is very good.
Previously I was addicted to the Black Knight Halberd but I didn't get it this playthrough.
Right now i'm about level 60 and on my way to being able to equip all the Havel's gear with the Havel's Ring. Right now I can only use 2 pieces to be a 50% weight burden.
Also..one thing i've noticed about this time is that there has been times i've died vs bosses within 2 minutes of entering, yet the 2nd try it's stupid easy. Don't ask me how but Gaping Dragon killed me on the first try within 2 minutes. Never thought this was possible. I somehow got stuck underneath him.