The block-out days, as pharmama said, make it a good idea to buy a regular pass and then upgrade if your trip happens to include a block-out day. You used to be able to buy block-out tickets for $40 if you had an annual pass, but that was recently eliminated and you now have to buy a whole 1-day park ticket for those. It's one reason I got a premium pass this year instead of a deluxe, and I'm still kind of miffed about it.
A downside to upgrading for some is that you can't use the monthly payment plan. They now have you pay the difference between your regular ticket and the annual pass upfront. If you live in CA and buy a pass directly (no upgrade), you can put money down--an amount that has to be at least equal to the cost of a 1-day, 1-park ticket--and divide the rest into 12 monthly payments with no interest. If you do this at the ticket booth, you can put down any amount you want. For instance, I put down $350 and did payments on the remainder. I'm not sure why they won't let you do that on an upgraded ticket. I think they used to, or at least I've read about others having done that in the past.