Hello Arno,
I could give you another interesting example - try searching for "BBY_DELETE_FORMS" in both legacy and launchpad search functionality.
Legacy search will return 3 notes - 2141687, 1300772, 1003308.
Launchpad search will return just 1 note - 2141687
Difference between those is that note 2141687 explicitly contains BBY_DELETE_FORMS keyword in note text under "Other Terms" section. On other hand notes 1300772 and 1003308 do not contain given keyword in text and thus are not found by launchpad search. Never the less they are relevant as they do contain correction instructions for include BBY_DELETE_FORMS.
It seems that unlike launchpad search, old search is taking into account actual correction instruction metadata - e.g. include, report, function, class, method names of objects contained in correction instructions.
And that was very useful feature to have. As SAP application support teams quite often identifies problems via debugger narrowing issue down to particular piece of SAP standard code. And afterwards SAP note search is made for all notes referring to given technical object (include, report, function, class). Would be great to have same functionality to be transferred to launchpad search as well... 
Regards,
Reinis