Like @NerfedFalcon said, there is a key item in the Asylum, but using it can leave you "stuck" if you're not ready. I'll spoil it for caution's sake the doll you find in the asylum gives you access to the Paint World of Ariamis in Anor Londo, but without the Lord Vessel to open bonfire teleportation, youre stuck there until you get past the boss..So I spent way more time than I needed to in Blighttown because I didnt't realize the big cobwebbed thing in the distance wasn't just part of the scenery, but the entrance to the big tiddy spidertaur boss fight. She was kinda easy (two tries). Really all of Blighttown wasn't half bad compared to Valley of Defilement. From memory that place was way bigger and considerably more unforgiving, with its huge swathes of poison shallows and all the ways the game would trick you into falling to your death. Blighttown is also surprisingly "generous" with its two bonfires.
I kinda miss the Demon's Souls style of progression where you'd go through these circular gauntlets that, without realizing it, would take you back to the beginning with a shortcut while also opening the way ahead. Areas in Dark Souls are connected in amuch more labyrinthine way, but the structure within each area itself is usually pretty linear.
@Xprimentyl @NerfedFalcon What's something good to do before heading to Sen's Fortress? I haven't cleared all of Darkroot Garden/Basin yet (made it to the Ents/Wargs after the Hydra; couldn't find the Golem). There's Demon's Ruins, Valley of Drakes, New Londo and Undead Asylum (where I think I did everything but beat Stray Demon).
But beating the Stray Demon nets you a much coveted Titanite Slab which, as stated, you won't need anytime soon, but is a precious resource.