This is actually very interesting. I have heard that Lenovos really dipped when they went over to the chiclet keyboard models. I will probably go out soon and try to find a very clean x220 that I can just put away until my current one dies.
They really did. More plastic and less magnesium seems like the most likely culprit. Luckily their desktops seemed to do that much later, if at all. I have the D20, which is their Xeon equivalent of the Mac Pro. It runs Mavericks so well that I haven't had to reboot in weeks. Easily the most stable computer I've ever had.