The book "The Final Hours of Portal 2" has a section where they explained how much pressure they felt to try and have Portal 2 live up to Portal 1. Even effectively shutting down development of of other games for 2 months so the whole company could focus on prototyping experiments for new gameplay concepts that could be incorporated into Portal 2.
I would not be surprised of the pressure they feel for Half-Life 3 to be great is an order of magnitude greater than what they felt with Portal 2