The first set of talents available to Death Knights are all about diseases.

Roiling Blood: Your Blood Boil now triggers Pestilence if it hits a target with one of your diseases on it.

This talent is especially nice for Blood-specced Death Knights (or tanks, if you prefer to look at it that way).  While Blood Boil won’t deal extra damage to the targets it hits unless they were already diseased, this makes it much easier to keep your diseases up on enemies when they keep joining the fight.  It also allows you to reserve your Unholy runes for Plague Strike and Death and Decay, so you can make Pestilence happen more often or more easily.  This gives it a lot of potential if you have many ways to turn your non-Blood runes into Death Runes, or if you just don’t want to worry about hitting that extra button to spread your diseases around.  This is slightly less useful for non-Blood Death Knights, but not by much.

Plague Leech (30 yd range, Instant cast, 25 second cooldown):
Consume your Blood Plague and Frost Fever diseases on a target to reactivate a random recharging rune as a Death Rune.

If you’re in need of runes a lot and don’t mind losing your diseases on your target, this can be a useful Talent.  One amusing thing to do with it is to power a Pestilence with the Death Rune it activates, refreshing the disease durations on your nearby targets and likely restoring the diseases to the target you just stripped them from.  While entertaining, this isn’t as strictly useful.  This power is best used to buy yourself a Death Rune to power a single-rune spell during combat, as it otherwise serves little purpose.  Death and Decay is an obvious choice early on if you need to keep ahold of aggro.

Unholy Blight (Instant cast, 1.5 minute cooldown): Every second for the next ten seconds, you infect all enemies within 10 yards with Blood Plague and Frost Fever.

If you rely on powers that consume diseases on your targets, or just find that you can’t get things diseased as often as you’d like, this can be a decent talent to pick up.  The fact that you don’t have to lay in your diseases for the start of a fight can really ease the load on your six-pack of runes, though the minute and a half cooldown keeps this talent from being as useful when you’re constantly moving along from one fight to the next.  This talent therefore has highly variable utility, depending on the situation you’re in.

Death Knight Tanks are most likely going to want Roiling Blood or Plague Leech, with a stronger tendency towards Boiling Blood- the damage from that is strong, especially at lower Death Knight levels, and the fact that it automatically strikes everything in reach and doesn’t give you another button to hit lets you focus more on managing your aggro.

DPS Death Knights will more likely find Plague Leech the stronger option- it can be triggered when a target is near the end of its life to either garner the needed rune to slay it before it can deal more damage, or to give you more ready runes for beating on the next target, depending on the fight.  Whether they prefer Roiling Blood or Unholy Blight for their damaging setup depends on whether they want an improved ability to deal damage while flinging their diseases around, or an occasional fight where spreading the diseases to begin with is just not an issue.