Please elaborate!

I have experience with several older Dell Latitudes (Exx00/Exx10 generations) and Precisions and found the build quality quite good.
Last year I bought a Thinkpad T430 and I must say I'm not impressed. It's not bad either but the build quality isn't as good as that of the Dells I know. For example the T430's display bezel isn't very stable and the designers really screwed up when positioning the USB3 ports. It's roughly on par with my old FSC Amilo Si 1520. Amilo is a consumer series.
I also know how a T60 looks like from the inside. It's built well, but not better than the Latitudes!

From HP I only really know the Pavilion DV6000. This is indeed a "plastic bomber" of the worst kind, but one shouldn't mix apples and oranges. If you want to compare IBM/Lenovo to Dell and HP then compare Thinkpads T/X with Latitudes/Precisions and EliteBooks. Or compare Thinkpads R/L with Vostros and ProBooks, or IdeaPads with Inspirons and Pavilions.
But don't compare series from different levels of "professionalism" (whatever that means).