The difference being the Shareware was much less obnoxious - you got the first act of Doom or Wolfenstein, and it was fully functional, you could enjoy it as much as you liked, the only impetus was to get more via shareware - modern F2P means that your experience is always hampered - even what you do experience is less than what it could be. Doom E1M1 was complete - you couldn't make the experience better by paying money, but there's always that XP potion or better item in a F2P model.