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.

This was an April Fools prank. Glad it got a few people.

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The Delta SkyMiles program has sunk to unimaginable lows. I thought it was ridiculous in February that Delta removed its award charts, so we would no longer know how much an award should cost. Now Delta has gone one step farther: Delta will no longer display how many redeemable SkyMiles you have in your account!

When the award chart was removed, Delta pointed to its improved award calendar as the reason a chart was no longer needed.

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.

Delta got rid of its award chart online.

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.

Delta had already released its 2015 award chart for travel beginning in the United States (exlcuding Hawaii) and Canada, and now Delta has released SkyMiles Award Charts for 2015 from all other parts of the world.

The current award chart controls for awards BOOKED in 2014, and the 2015 chart for awards BOOKED in 2015, regardless of when the flights are flown.

In addition to new award charts, we will see two major rule changes on Delta awards for awards booked January 1, 2015 or later:

Delta will allow one way awards for half the price of roundtrips
Delta will