I'm of the opinion Valve's work will change neither for the better or for the worse as a result of this deal.
1) EA Partners IS NOT EA (atleast in the sense you're thinking of.) EA Partners allows you a lot of creative control, even over your marketing and stuff. You're ultimately responsible, and they spend very little. It's low risk because it's ALWAYS on talent they can bank on. Valve is extremely bankable.
2) Valve isn't infallable. In my opinion (and I might be alone) the quality of half life 2 wasn't up to what half life 1 was. It had amazing art design, but it ended there for me. I found the story to be non existant, and they took no chances.
3) Why would EA care about Half Life 3,4,5? They know 1 and 2 were super sellers, it's not their property, they don't own it. They won't even bat an eyelash to change what's there. They only destroy good stuff when they own it and they do it in house. Neither of these is true. (Refer to Criterion's continuing quality on the Burnout series and the success of their renderware engine.)
4) I love pudding.