"When buying a Macbook, I know I'll be able to get parts and service for it - often from a local Apple store - for several years. If I buy a Dell N5040 today, and need service for it 2 years from now, will I be able to get it?"
There's one other thing you're getting: resale value. I wrote a piece on how to buy a Mac: http://jseliger.com/2010/11/01/how-to-buy-a-mac/ and should probably write another on how and why to sell them. I first bought a PowerBook in 2004 and sold it for about 40% of its original value three years later, at which point I bought an iMac, which I sold about a year ago for half its value. My parents were amazed that computers could hold any kind of value at all; part of the reason Macs retain their value is because Apple doesn't sell really cheap Macs, but the other part is simple: buyers can actually figure out a) what configuration you're selling, b) its approximate age, and c) its approximate value. If you're selling a random Dell or HP with a string of numbers and letters after it, most people can't tell if you're selling a computer you bought six months ago for $3,000 or a computer you bought six years ago for $400. You can reinstall the OS quickly and easily.
Basically, no one save (to some degree) Lenovo with Thinkpads has any kind of brand recognition.
The extra one that should be mentioned (but only for used Macs, I suppose) is "what was the latest Mac Apple dropped support for in OS X, and how much newer is the Mac you're looking at?" I hear with Lion they burned the Intel Core Duo (i.e. 32-bit) CPU Macs. Which are probably still pretty solid machines if you get some other OS running on them.
There's one other thing you're getting: resale value. I wrote a piece on how to buy a Mac: http://jseliger.com/2010/11/01/how-to-buy-a-mac/ and should probably write another on how and why to sell them. I first bought a PowerBook in 2004 and sold it for about 40% of its original value three years later, at which point I bought an iMac, which I sold about a year ago for half its value. My parents were amazed that computers could hold any kind of value at all; part of the reason Macs retain their value is because Apple doesn't sell really cheap Macs, but the other part is simple: buyers can actually figure out a) what configuration you're selling, b) its approximate age, and c) its approximate value. If you're selling a random Dell or HP with a string of numbers and letters after it, most people can't tell if you're selling a computer you bought six months ago for $3,000 or a computer you bought six years ago for $400. You can reinstall the OS quickly and easily.
Basically, no one save (to some degree) Lenovo with Thinkpads has any kind of brand recognition.