but take for example my Address book
Address Book comes with the OS and an Intel-capable version will be already on the new Mac when you start it up.
We recently migrated my wife from an eMac running 10.4.11 to a new iMac with 10.8. Didn't use Migration Assisten, rather I made sure all her data files were in her Documents folder and copied it (over the home network) to the new computer's desktop, then figured out what needed to be moved. Address Book was easy--it has an "Import" command under "File" that quickly and accurately transferred all her contacts.
Yes--data files are personal files you have created using programs on your computer or gathered for your personal use from other sources (e-mails with files attached or something downloaded). Data files are unique to your computer. Think of them as a collection of artwork you created. If you buy a new studio, you take the art with you, but you would leave behind the lights and the plumbing because they are hard to move and your new studio will have lights and plumbing anyway.
Program files, support files, and preference files are best left to the newer OS to handle. Some programs will come preloaded on a new Mac. Mail. iPhoto, iMovie, Address Book, etc, and will be the latest versions. For the majority of these, you can move data ("your") files created with one of them to the new computer and the new program version will open files you created on the old one quite nicely.
For non-Apple applications you own, you'll need to look at the specs and see if they run on Intel Macs--a few do. Microsoft Office 2008 ran on either Intel (2006 through today) or PowerPC (pre-2006) Macs. Until the 2012 tax year, TurboTax would run on either. If the specs on the box or the program's web site do not say they have Intel support, you'll either need to check the developer's website to see if they have released a patch to make the application Intel-capable, or buy a newer version.
I had an extra copy of Office 2008, so that went on my wife's new iMac right away; her data files I moved all opened fine even though created on a much older version of Office. She also had years of TurboTax application programs and data files. All but three were released in years when TurboTax had support for both Intel and PowerPC, so I moved them. The three years of TurboTax program and data files that predated Intel Macs ended up on a PowerPC minitower I keep stuck back for running an older version of Adobe Creative Suite.
Also, when it come to copying 'over the home network' did you plug each computer into the router?
Yes, I used the router we have on the home network. We have four Macs on the wire and two Mac notebooks connecting wirelessly. That eliminated the concern about needing special cables. Older Macs required not the regular ethernet cable but a "cross-over" ethernet cable for a direct computer-to-computer transfer. I regret I cannot remember when that requirement disappeared. Going through the router takes that old sausage out of the equation.