Enchants would be a huge buff to you as from a glance it didn't apear you had any. They also show people you are putting your best foot forward with gear, someone without enchants to me screams lazy. If I knew DK specifics I would try to be more specific however I will say stamina/defense enchants are a good start and I am sure there are solid in depth DK guides on here if you look around a bit.
I've had it planned to enchant my gear. Currently I only have my chest, back, and hands enchanted. I've been trying to hold off on enchanting till I get gear that i'll be keeping for a while as I don't want to spend too much gold on something I might not be keeping for long. I'll see what I can do about some stam enchants and raise my rep with Argent and Sons of Hodir for their gear enchants. I've read quite a few of the DK guides here but most of the ones that i've read are for gearing and not builds/enchanting. I'll look into it though.
Gear wise, you're fine to tank some easier heroics. I would like to recommend a few things though. Surprisingly you are uncritable without stoneskin gargoyle, which has it's advantage, you can use swordshattering, fallen crusader doesn't help tanks much at all. You should look into getting your defense down and grab stoneskin since the +2% stam is nice. To help with this I'd start with getting rid of daunting legplates, greb Special issue legplates from Argent crusade or hit H VH (an easier heroic) for Bolstered Legplates. The rep with argent crusade as well as sons of hodir is a good way to go for the enchants too. You should also grab frosthide leg armor. Finally, essence of gossamer is an amazing trinket from H AN that you should definately get.
Now on terms of your spec, dk's have more freedom than any other tanking class, but there are limits. I won't tell you what to spec exactly, it's your choice, but a few things to know are:
1. blade barrier/toughness/anticipation are mandatory
2. try not to spec to deeply into a tree, 51-52 points is plenty
3. know what each tree is best at, different trees respond better to different stats, (blood benifets from EH better than the other two trees, while unholy is mostly to be the avoidance tree)
4. While you are fine now, make sure you always have your spec's respective cd, which you do atm, but make sure you keep it
Also, on the topic of spec's, you have gargoyle, which is cool for dps, but doesn't add threat to you as a tank, so it should be skipped.
I chose not to go with stoneskin gargoyle because I read on certain guides that it raises your defense but not your rating. Even though your def is 540, you're not uncritable. I'm going to try to replace some of my lower level blues for some epics and respec.
I dont know where you read that, but if you read up on the guides here, and on EJ. Your uncrittable value is 540 defense, no matter how you get it, pure defense or defense rating. By going with SSG it will allow you to grab lower defense gear with better TV/MV stats.