One of the biggest things for me is the Dell keyboard. Since I spend so much time on the keyboard, I'm pretty finicky about it, hence the choice of ThinkPad.
They've gotten a lot better. The Latitude D600s had horrible keyboards that just felt cheap; not secured strongly to the base and all sorts of creaks and such after a year or so.
However they rectified that problem quickly. All the x10 and higher use multi-post mounting for the keyboard, solid as a rock. I've typed on a Lenovo before, I'm posting this right now from a Dell Latitude D820 and it feels almost exactly the same as Lenovo's.
No offence to the other posters but Dell licks ballz I have never and will never buy one they break down so easy only computer worse then a dell is a emachine (absolute worst).
I can not stress this enough DON'T BUY A DELL!!! Unless your going to get one of their very top gaming machines which are 3k+ I might trust one of those but why when you can get a Alienware loaded for that and not have to worry about it.
This I can't agree with at all, different experiences I guess. In my 3 years here I've seen about 200 Dell notebooks come and go, they get abused pretty hard by the morons that bring them to mine sites and generally don't give a shit what happens to them.
Anyways, our notebooks see a lot of abuse. They break down, I've had to replace a couple of motherboards, half a dozen hard drives, and at least 2 dozen keyboards. But compared to the number of notebooks we have deployed, I am beyond impressed with the stability of modern Dells.
I used to recommend HP to people who asked, because they were about as stable as Dells but much prettier. Dell's got a good design team now though, and the flawed looks in the past are long gone. I push Dell's name every time someone asks me for notebook recommendations nowadays, because time and time again I've been impressed by their systems.
Support, that I couldn't answer. We buy the extended warranty on all systems and basically get overnight parts replacement, but it is an extra couple hundred bucks you can overlook. Their standard support is, from what I've heard, less than phenomenal.