HL1 was so awesome because of what happened with the community. MODs and the like gave new life to the game in every aspect.
HL1 single player was fucking amazing, I still remeber how pumped I was after finishing the demo and running to walmart at 3am to buy it for $54.99. That was a year after its release, when the MP portion was up to par. I remember reading in the manual that Valve would give you a full retain refund if you didn't enjoy the game, for the first time in my life (for a purchase over $20) I threw away the receipt, I believed in the game that much.
This "suprise" had better be HL2DM, I can understand them not releasing it with the retail, it kinda ruins the SP experience. Jumping in MP and seeing/using all the guns ruined the game kinda for HL1, I jumped in MP about 1/2 through the SP game and I was blown away by the rocket launcher and crossbow, I hadn't found them yet in the SP and would have loved to not have seen them yet.