Define "serious difference".
Integrated circuit chips plugged into sockets, or soldered directly, to boards... Resistors... capacitors... diodes... SATA hard drives from Western Digital or some other manufacturer; DDR or whatever RAM from whatever manufacturer; ATI Radeon or NVIDIA graphics boards... 10/100/1000 baseT copper ethernet, 802.11g or n, standard USB, standard FireWire... Pretty much many of the same items you'll find in any higher-end computer...
The differences will be in the firmware in those ICs, and of course, the operating system you choose to run on that hardware.
Just like you can't make your own HP/Compaq or Toshiba or Sony or Dell or whatever other name brand computer out there - BIOS and firmware and minor tweaks make each manufacturer different, you can't "make" your own Apple Macintosh. Yes, you can "make" your own computer and run an operating system on it - Linux or some other form or Unix - or Windows... Apple's Operating System licensing and IPR in the ICs does not allow you to install MacOSX on just any old machine.
So... when you get a MacBook Pro, what exactly are you wanting to do to it? Install RAM? Replace the hard drive? Laptops - Apple or otherwise - have never been "tinker-friendly"... Same with all-in-one machines (iMacs or HPs or Dells)... "Tinker friendly" machines are typically towers with card slots... but generally speaking, it is the "secret sauce" on the motherboards and the CPUs + the OS that make the machine different... Remember, an Intel Core Duo or is still an Intel Core Duo... The manufacturers have to have a competitive edge, and that generally comes in the way of either price or features.
Just like comparing cars... A Kia has pretty much the same "things" a Cadillac has... air conditioner compressor and vents, seats, sound system, motor, tires, alternator, fuel injector, etc... but one costs more and, for the most part, the parts are not interchangeable... and just because you know all about one does not mean you know all about the other. It is the differences - price and features - that appeal to different people.
By the way, I used to be an Information Technology (IT) manager with a crew of 6 folks and we responsible for ~500 Windows and ~500 Macs and all the LAN/WAN interconnectivity between 13 offices in California... In my current gig, my company makes me use an HP/Compaq laptop running Windows XP - but I have always spent my own hard-earned money on Apple Macintosh computers. Currently, family members have a G5 flatpanel iMac, a G4 tower, a CoreDuo flatpanel iMac and a G4 laptop... and the HP/Compaq nc6220... all happily sharing the LAN and DSL to the internet.