I am by no means a "leet" player, but I do love playing my DK tank. My first suggestion is actually to get glyphs. You have one glyph, and imo it is not a strong glyph.
Obviously you should fill all of them, but the ones I mentioned are a good place to start.
You mentioned your gems, so you know to fix that, along with enchanting your gear.
It looks like you are very aoe specced, I assume that is because you spend most of your time grinding dungeons. Personally I don't see an issue with that, but if/when you progress further you should take a look at the Blood DK thread on this forum. It would actually answer all the questions you have asked here, and the answers would be from some very good players.
You should also pick up some Professions and max them out. I don't know the "best" professions for tanks, I have BS and Alch on mine, which gives me a means of making some gold and an extra 80 mastery from the two extra sockets and a nice mastery trinket in the Stone.
You really should check out the Blood DK thread, it might take you an hour to go through it, but it is very informative, it helped me a bunch.