The very first time I specced my Death Knight, I went Unholy. It did not last.
Back in Wrath, all three DK specs could both tank and DPS - you'd have to use a different talent layout, of course, which led me to spec my DK as Blood/Blood for much of the expansion after the introduction of dual-specs.
I loved playing Blood DPS, though in Wrath I went a little tank-crazy and actually played all four of the then-available tanking classes as tanks.
Still, Blood really fit the fantasy of the Death Knight as I conceived it - a big, imposing runeweapon slashing at foes and following up with bursts of shadow damage.
When Blood was made exclusively a tanking spec, I still stuck with it because my Death Knight was still a mainspec tank. But for soloing I went with Frost, largely because Frost was built around these massive Obliterate crits.
The thing is, while I wasn't drawn to the pet management of Unholy or the awkward focus on disease spreading, I did enjoy the fact that Blood and Frost still did raise ghouls to fight for them - it's just that they were fire-and-forget guardians that the spec wasn't really built around.
Now, however, a lot of the things that made me mainspec Frost for the past two expansions have gone away. You can't use a two-handed weapon. You don't have any dead-raising abilities (except for your battle rez.) Obliterate does still crit big, but you're never going to see those massive numbers anymore because it's divided between your two weapons.
Meanwhile, Unholy has lost a lot of the things I didn't like so much. Dark Transformation has been simplified into an easily-managed 1-minute cooldown (that's a bit more impressive-looking than Pillar of Frost.) And disease spreading is way simpler with the redesigned Outbreak.
Instead of managing diseases like some kind of melee affliction warlock (ok, it was never that bad,) instead you now get the Festering Wound mechanic, which is pretty unlike anything I've seen other specs deal with.
And you can replace your ghoul with an abomination, which is pretty awesome (though holy crap do they need to update the abomination models - not redesign, just a warlock-demon-style update.)
The fact that Frost gets swords made from the shards of Frostmourne is making me seriously question this decision, but I think that ultimately, Unholy might fit me better these days (unless I wind up going back to Blood.)