Delta has introduced revenue-based awards today and higher priced upgrades for travel beyond June 1, 2016. You can currently book award travel and upgrades through June 9, 2016.

Delta's Statement
Here's what Delta says about the changes:
We know your miles are important, so we want to provide the most notice possible regarding Award price changes. For travel on or after June 1, 2016, the number of miles needed will change based on destination, demand and other considerations. But most Award prices will remain unchanged.

According to Delta's SkyMiles FAQs, mixed cabin awards are no longer allowed.

Mixed cabin awards used to be explicitly allowed (and indeed this FAQ at a slightly different URL has the old language.)

If mixed cabin awards were totally banned, that would terrible news. I can see three possible maladies:

Delta has "five" cabins: economy, Business, Business Elite, First, and One. Each plane has only two cabins but the front cabin is branded several ways.

Wandering Aramean got "part of an internal memo describing [United's] plans for the coming year." It says:
Deliver first phase of united.com 2.0 by 2q 2015, including bundled ancillary offerings;
Some time in this quarter (by June 30), United plans to roll out a new website with new bundled fare options, which sounds like what American Airlines offers. Not that interesting. Moving on...
begin introducing dynamic award pricing.
Uh-oh!

"Dynamic award pricing" would mean that award prices change, presumably based on the cash price of the underlying ticket.

Wandering Aramean got "part of an internal memo describing [United's] plans for the coming year." It says:
Deliver first phase of united.com 2.0 by 2q 2015, including bundled ancillary offerings;
Some time in this quarter (by June 30), United plans to roll out a new website with new bundled fare options, which sounds like what American Airlines offers. Not that interesting. Moving on...
begin introducing dynamic award pricing.
Uh-oh!

"Dynamic award pricing" would mean that award prices change, presumably based on the cash price of the underlying ticket.

I was explaining to my brother the changes that Delta has made in 2015, and I mentioned that the one I just couldn't believe was that Delta eliminated its award chart. It seems self-evident to all of us in the miles game that Delta eliminating its award chart is insane and unjustifiable.

(Delta deleted the chart from its site, but the possible priced for awards are the same now as they were when the chart was on delta.com.

I was explaining to my brother the changes that Delta has made in 2015, and I mentioned that the one I just couldn't believe was that Delta eliminated its award chart. It seems self-evident to all of us in the miles game that Delta eliminating its award chart is insane and unjustifiable.

(Delta deleted the chart from its site, but the possible priced for awards are the same now as they were when the chart was on delta.com.

Southwest currently has a purely fixed-value rewards program. For every dollar of the base fare, you have to redeem 70 Rapid Rewards to get your ticket (plus you pay a $5.60 per direction government tax on each redemption.)

That's about to get worse, much worse in my opinion.
Current Rapid Rewards Scheme
Let's look at an example. Here are the pries for tickets from Los Angeles to Tampa this March. They vary greatly by day from $207 to $334 to $607.

If you select a day with $211 fares, not all fares that day are $211.

I gathered several estimates for the timing and extent of the next American Airlines devaluation from miles and points experts. I've averaged the guesses to produce a wisdom-of-the-crowd estimate that I hope is as close as possible to predicting the future as we can get.
Background
Last year, United gutted and bifurcated its award chart for Business and First Class redemptions. Now awards on partner airlines cost a lot more than they used to. For instance, one way in Partner First Class to the Middle East is 140,000 miles.

Delta released a few new award charts last year.

As widely predicted, Delta is eliminating stopovers on its awards starting January 1, 2015. Its SkyMiles program rules page says as much.

This is a bummer, but the good news is that one way awards will be bookable with Delta miles starting January 1, 2015, and Delta has already improved its online search.

American eliminated stopovers on awards this year, and the US Airways program will cease to exist in quarter 2 of 2015, which means United will be the only American program that allows stopovers very soon.