For me it's the backtracking. Especially if your first pass through affects what's going to be there when you go back.
That's why I hate Hard Rain campaign in Left 4 Dead 2 -- every health pack, bottle of pills, adrenaline shot, defibilator, or ammo packs have to last you both ways. It's not the regular Left 4 Dead feeling of 'Here's a health pack, I'm low on health, fuck yes, now I have some health'. It's now 'Here's a health pack, I'm low on health, but I should leave this here for future me'. It adds this high level of stress and the whole "but I might need it later" syndrome.
That's why I hate Hard Rain campaign in Left 4 Dead 2 -- every health pack, bottle of pills, adrenaline shot, defibilator, or ammo packs have to last you both ways. It's not the regular Left 4 Dead feeling of 'Here's a health pack, I'm low on health, fuck yes, now I have some health'. It's now 'Here's a health pack, I'm low on health, but I should leave this here for future me'. It adds this high level of stress and the whole "but I might need it later" syndrome.