Ghostcrawler explains the philosophy behind the DK changes as:

* Making tanking still rely on cooldowns, but rely on them a little less.
* Make Blood a more attractive spec, and particularly for tanking.
* Chill out some of the defensive capabilities all DKs have in PvP.
* Avoid having to use Death and Decay as an out-of-combat runic power generator.

You can read the full list of planned changes here.

Blizzard poster Nerimash explains that the Mage changes:

are designed primarily to make Arcane more competitive with Fire / Frostfire builds in PvE. Because they could end up buffing Arcane in PvP, we are prepared to adjust the damage of say Arcane Barrage if needed, but we want to see these in action on the PTR first.

Highlights include:

Evocation – cooldown reduced to 4 min.
• Arcane Flows – now also reduces the cooldown of Evocation by an additional 1 / 2 min.
• Arcane Blast – overhauled. Will now increase the damage of your next Arcane spell by 30%. However using Arcane Blast itself does not consume the charge, but instead increases the mana of your next Arcane Blast, up to a maximum of 3 stacks. You can alternate Blast and Barrage to keep buffing Barrage, or you can build Blast up higher for a heavy mana cost.
• Torment the Weak – now works with Arcane Blast and does bonus damage to targets afflicted by any kind of slowing effect (e.g. Thunder Clap).
• Elemental Precision – renamed Precision and now works on all spells.
• Improved Blizzard – snaring effect reduced to 20/40/50%.

Official Mage post is here.

Official Death Knight post is here.

the death knight really suffers when attacks fail to land, especially opening attacks. Having a Plague Strike dodged or parried can throw off an entire rotation, eventually leading to bad Obliterates because both diseases weren’t up at the same time. Missing attacks is a fact of life for all tanks since it’s hard to stack +hit and +expertise along with mitigation stats. However, existing tank classes have mechanics to let them “burn off rage” (or mana) to make up for those bad strings. The DK plays more like a rogue who can’t get enough combo points in play. This had a deadly effect on DK threat generation, which just didn’t compete with that of other tanks.

He then goes on to list 5 key changes to DK abilities:

You can read the full post with some more explanations on the above changes here.

Highlights include:

Q: Is there any plans to give us spam able Aggro-generation skills back?

A: Abilities that add too much straight threat don’t scale well. Paladins would have been far and away the best tanks for threat in Sunwell if more gear had been available to them. Part of the reason is because their threat scales so well with their dps. We’d like to get away more from +threat abilities for all classes if we can.

Q: Are we supposed to be hitting the Armor Cap? Or an Avoidance cap? Because right now i do not see that happening either way and leaving us kind of… lacking mitigation.

A: No class should be close to either in Lich King. That was actually a problem in Burning Crusade — witness bears being unable to improve their tanking gear because of the armor cap and the introduction of “Sunwell Radiance” to work around excessively high avoidance values. We think it’s fine for tanks to reach the uncrittable mark, and being uncrushable is no longer an issue. Beyond that, we want you to always have a reason to improve your gear… and always be a little scared of those bosses.

You can read the entire post here.