For Bioshock, it's everything after Andrew Ryan dies. The game just kind took a nosedive after that, and I think I may have only actually finished it two times. It's similar with Fallout 3, where I try to put off going through that stupid ending as much as possible, so I generally end up not finishing most of the playthroughs I start.
Then there's the section at the beginning of Half-Life 2 that has you slogging on foot through canals and sewers, which is really quite dull. The game picks up a great deal after it's all said and done, but it's boring and a bit hard to get past once you've done it before. It's why I usually just stick to the shorter-but-sweeter Episodes when I need my Half-Life fix.
Then there's the section at the beginning of Half-Life 2 that has you slogging on foot through canals and sewers, which is really quite dull. The game picks up a great deal after it's all said and done, but it's boring and a bit hard to get past once you've done it before. It's why I usually just stick to the shorter-but-sweeter Episodes when I need my Half-Life fix.