I have asked the author of the test about the missing songs and after checking again, he now changed the number to five. It seems that in an older version of the import tool there were more songs missing, but in the current version all songs from the original Rocksmith disc can be imported, except for:
Eric Clapton - Run Back to Your Side
Cream - Sunshine of Your Love
Seth Chapla - Star Spangled Banner
Seth Chapla - Jules
Chris Lee - Boss
While this information is confirmed for the German version, I cannot guaranteed that there are no differences in other countries, but the list of missing song goes in accordance with the English FAQ of Rocksmith 2014, that says that...
..."you will be able to import 52 of the original songs into the new Rocksmith 2014 Edition [...] except Eric Clapton "Run Back To Your Side" and Cream "Sunshine of Your Love" from the main disc tracks, plus 3 of the 5 unlockable bonus songs."
To import the songs from the Rocksmith disc, you need to have a save file of the original game on your hard drive. When first starting Rocksmith 2014, you will be asked if you have played Rocksmith before, to which you must answer "yes". On Playstation 3 and Xbox 360 you will get access to the import tool in the ingame store under the tab "packs". It does not show up, if you answer the question with "no".
On PC, the import tool can be bought at any time from the Steam Store.
According to the German Ubisoft support (last paragraph), if you answered with "no" on the Xbox 360 version, you can only get access to the import tool by moving the Rocksmith 2014 save file onto a USB or external hard drive. When starting the game without a save file on the hard drive, the question should appear again and you can download the import tool. After that, transfer your save files back.
Please note that in the Ubisoft support entry, this has only been confirmed for Xbox 360, although it is likely to work for the Playstation 3 as well. This question has not been answered on the English Ubisoft support forum, which is why the link leads to the German one.
Also, all DLC songs from the original Rocksmith can be used in Rocksmith 2014. For the PS3 you can download them from the PSN by searching for Add-Ons for the original Rocksmith. The DLC for the original Rocksmith does not show up in the Add-On list of Rocksmith 2014 in the Playstation Store, but after downloading and installing the songs, they are automatically recognized and added to your song list. You do not need the original Rocksmith to play any DLC songs in Rocksmith 2014. This is likely to work the same way on the Xbox and PC versions.