Uhm, the character development and revealing of mysteries and clues as to the whole truth behind Half-Life is awesome. There are still unique and mysterious things being figured out about Half-Life 2. Half-Life 3 is also in the works and has been for a long time now. I personally would love it if hey stretched it to Half-Life 10 but 3 is probably as far as it's going to go.
One of the things being theorized right now is that someone on another forum noticed that the Zombies in Ravenholm say "Help me" in Danish to you when they scream. Another thing is that in the beginning when you walk around City 17 and the Combine are busting into people's apartments, if you walk in the playground you can hear the voices of young children. There's a bunch of stuff on Wikipedia too from people who analyzed the sound files in Half-Life 1 and figured, for the most part, what was being said between the G-Man and the scientist in Black Mesa in the beginning, among so much more.
IMHO, the Half-Life Saga is the most beautifully done FPS out there and I can fully appreciate it, despite Valve's release delays, but they just want to make sure it is **** good.