Alaska Airlines announced a number of changes today. There has been growing competition in the Seattle and West Coast markets as Delta has increased their flights and community involvement. Up to now, Delta and Alaska had a code sharing agreement, and you could earn miles on the other carrier by flying the other, earn miles for Alaska on Delta and vice Versace. Delta nerfed those earning rates a couple years ago, but you still could get something in Alaska miles on Delta.
With the announcement today Alaska is lowering their minimum miles on some routes to 5,000 miles one-way, and appears to be strengthening their arrangements with other non-US carriers to bulk up their international partnerships.
But the ending of the code-share and miles bridge means the gloves are off in Seattle between Delta and Alaska. It's been good for us who fly Delta out of Seattle, and this healthy competition should keep things interesting and good deals for some time to come.
I've always prefered Alaska. Living on the West Coast it was almost always the cheapest for me and has the best mileage plan in my opinion. I've never been a big Delta fan so I won't miss the Delta alliance. I've been flying American quite a bit recently as they tend to have more routes from Reno for my work trips and I can still get Alaska miles. Sadly, the nerfing of American earnings on Alaska will leave me a few thousand miles short of MVP Gold this year 
I think the participants who prefer one or the other, Delta vs Alaska, will do fine by keeping their loyalty in that carrier. The folks who were enjoying being able to play both Delta and Alaska together will be the most disappointed, will need to choose one.
For the work travel I do, Delta is clearly the better choice, so I'm already less of an Alaska Air fan. Others like Alaska, and that's fine.