Browsing through Sony's pre-1979 catalogs makes one feel this wasn't all that new: there were portable tape players aplenty and many had about the same size and looks...
The TC-44 for instance or even the TC-50 that went high up into space ten years before the TPS-L2 was released.
What probably did it, besides weight (300g, as opposed to the 900g of a TC-44), market-wise, is that none of the previous portable cassette players were tagged WALKMAN - more a matter of concept than anything else, in proper Sony tradition.
Case in point I : the TCM-100 was a recorder released in april 1978 - same size, same weight, same design but it wasn't named Walkman...
Case in point II : despite the TPS-L2's commercial and sociological success, the real masterpiece is the later WM-II... and it was originally designed at the time of the TCM-100, more than a year before the TPS-L2.
What mainly separates the TPS-L2 from its less successful sibling is the "talk-back" orange button (to reinstate possible discussion with other people) and the two headphone jacks (to initiate love-music listening with another one person, at the exclusion of all others - those boring and generally older ones who never do understand what Weltschmerz is all about).
Several monikers and associated logos were tampered with, "Soundabout" being the most well-known, but the traditional WM code didn't come with the first Walkman because although it was planned to be a success, it wasn't expected to become such a success.
The TPS-L2 was rebadged as a WM-3 in 1981 : same object but black, with the WALKMAN logo on the lid, success oblige.
It should be noted, however, that only the early samples of the TPS-L2 didn't have that logo - quick success oblige !
I remember borrowing my cousins' new gadget early in 1980 - I felt like a super-hip dude.