In this page i will be briefly going over each talent tree and giving recommendations to what kinda player you are to the tree that you should pick because of your play stile. The Death Knight is made with 3 talent trees Blood, Frost and unholy. All special in there own way. Blood is a tanking tree that has good health regaining abilities useful for tanking. Frost is a dps tree that uses frost spell and attacks to land nasty damage. unholy is also a dps talent tree that uses disuses and damage of time spell to kill things. Depending on what kinda player you are will decide which talent tree you will want to be.
Blood is a tanking spec that uses abilities to regain and take damage awhile keeping good threat with decent damage.If you are a fan of tanks and always have been then this is the tree for you. It is very good at soloing quest that you will sometimes need 3-5 players on. You can also kill 2 or more targets with its good damage taking abilities and healing.also really good for gearing up because of instant ques in to heroics.
Frost is a dps spec that uses frost spells and attacks to deal large amounts of damage. If you have played a warrior or a ret pally this is most likely the spec for you. This tree is also very good in leveling because you get some serviceability talents too with your damage. This also has very good hast beacsue of the talent Icy Talons witch also helps raid members so it can get you in to that raid that your not really geared enough but they want your buff.
Unholy is also a dps spec that uses diseases and pets to deal a lot of there damage. This tree is compared to a warlock a lot because of there ability to stack DOT (Damage over Time) witch can deal a hole lot of damage if keeping it up constantly. Unholy also has a permeate goal that follows you and deals a very good bit of damage if you glyph it. If you are a fan of warlocks or had one in the past this is most likely the spec for you.
To conclude this page im just going to say that it doesn't really matter what tree you pick it just matters if you are having fun and its not like its a lifetime decision you can always change any day if you are not satisfied.