- the connector for AC adapter and/or the adapter itself;
- a random key on the keyboard;
- the battery.
I keep replacing parts, but eventually I buy a new laptop because the current one seems to have seen too many wars.
Only once (HP laptop, AMD processor) did it really go beyond: the hard drive, then a couple of months later, the motherboard, so ill-designed it died from overheating.
I guess all that says a lot about how I use my laptops: I travel a lot, to dusty places with bad electricity supply, and use them 14 hours a day (eating, drinking, smoking). Will soon have to replace the wetware as well, I guess.