Unholy is probably one of my favourite specs in the game. The theme of a dark knight controlling diseases and having necromantic spells is brilliant and the Outbreak/Epidemic combo is a very unique cleaving style. It is not easy to play compared to say a BM Hunter with it's 3-4 buttons rotation but that is one of the things that makes me want to play it even more.
Frost atm falls a bit short for me even though the dps is supposedly great. Playing with Sindragosa may be fun for some people but I feel the downtime is too long between breathes and I also find the ST damage a bit lacking compared to Unholy that can burst ST and groups just as well. Not to mention that a DK with two little swords is just not fitting the class fantasy, especially since you are doing heavy swings and cleaves... anyway...
I learned after some trying out that one thing that really gets a DK going, both in Blood and Unholy is Haste. A base 32 to 34% upwards makes things A Lot smoother and rune regeneration is much better. I take great pains not to fall below 30%, even if that means using lower ilvl rings.
Below a certain Haste threshold Blood felt nearly impossible to play since you ran out of runes, then runic power, then health, after crossing the threshold the palystyle becomes so much smoother. The same goes for Unholy pretty much, just that you won't be murdered by a group of trash and instead have longer and unfun downtimes in your rotation.
And as has been said, make liberal use of runic power, Death Coil for ST and Epidemic for Aoe, the added benefit besides Runic Corruption is that you reduce the CD of your Dark Transformation (which is already pretty short to begin with).
Since most of our CDs are pretty short, don't hold them back. I made that mistake quite a bit in the past. I generally just throw the DT on whatever is pulled and then usually spend my first 4 wounds on a juicy Apocalypse. Then you could go into Unholy Frenzy to get new wounds quicker and for free and just Scrouge Strike a bit until you have enough Runic Power to cast Death Coil/Epidemic for Runic Corruption (also using up Sudden Doom procs). Once you are low on Runes AND Runic Power (for example if you did not get a Runic Corruption proc) you go for Soul Reaper to pass the time before you can hit another CD.
We have alot of CDs that want to be pressed, which is why I am glad that Vision of Perfection is doing so well for us atm, not having another CD to watch allows you to focus more on the class abilities.
Finally Army of the Dead is our biggest baddest move. Unless you are fighting an overpowered plotshielded Banshee it is just a joy to press that button (one of the reasons the spec is so fun to play), you become a one-person army for 40 seconds, with 3x Magus of the Dead making it even better.
It goes without saying that you want to line it up with Bloodlust/Hero in M+ and Raids. If the Raid uses Hero at the start of the fight you can cheat a bit by using it 4-5 seconds before the pull, getting a few seconds of rune recovery to start the fight with (nearly) full runes and an army of skelletons at your side for the next halve minute.