I will withhold judgment on Call of Duty for now, but six months ago I would have told you that it needs to die. Apparently CoD4 is new and innovative and really great and all that shit. 2 and 3 were just hollow re-treads of an amazing original that they just couldn't ever live up to.
Battlefield is done for me. 1942 was groundbreaking, original, and great fun; Vietnam just couldn't make up for the glaring weaknesses of the engine (When I click an icon on my desktop, it is because I want to play the game, not because I want to wait for a bunch of vanity developer/publisher logos, a bloated menu, and a horrific load time on what was a pretty decent computer at the time). Battlefield 2 improved on this, although it didn't fix the load times, but it didn't add a lot of new things, and despite the promise of 2142's premise, it didn't deliver on innovation. The developments in this series are becoming minor alterations to a gameplay format that's getting old, and which have always been unable to hide the glaring programming issues. If you have paid somebody to program in a feature that pops up a box on the screen that says "loading menu" after they hit esc, your development priorities are in the wrong place.
I am not done with Half-Life; every one has been well worth playing, and I always look forward to the next iteration. Valve is very concerned with the quality of their IP; when Half-Life jumps the shark, they'll know, and they'll end it.
No One Lives Forever has been ended, and though the older games are a little bit dated and have a bunch of irritating quirks, I think one last well-done game to cap off the series is overdue.