You're sufficiently geared to start heroic-ing it up. Not impressively, sure, but enough. You could add a bunch of enchants or grab a belt buckle, but that's plenty.
Best advice would be to look at swapping out trinkets. Crit rating's just not that important. You can get more stam than the darkmoon card too...
Your real problem is that with your current kit, most of the upgrades you can get or earn will reduce your def below uncrittable. You can improve that some by dropping enchants, concentrating on the gear you'll be keeping for a while (the boots, for example).
You'd also do well to take a short break and jam on your professions. You're getting no benefit at all from your engineering as it is, and leveling mining will make it easier to get the materials for crafted gear (not to mention fattening your wallet, and DIRECTLY increasing your HP! ;p)
People are just used to running heroics with people wearing three or four pieces of raid-quality epics. They aren't necessary in Lich King, not if you know what you're doing.
Right now im sitting on 20k hp unbuffed 20k armor 541 defense 15.86% dodge 15.18% parry and 15.56% block and people tell me im still undergeard for heroics. So my question is WTF am I doing wrong?
With the sounds of your stats, you should be fine to begin heroics. However, your stats are not ideal. You'll want to check wowhead and see if there are any significant upgrades you can get to your gear, and try to grab them. Then buy the cheapest enchants you can (if money is your problem) and try to buff your health up a bit more.
I can't see your armory but I can suggest grinding Wyrmrest Accord to Revered for the 3 pieces of tank gear.
Pel, I was very recently in a very similar situation....almost identical stats. I didn't look at your armory page, but I suspect gear quite similar. And I didn't want to blow a lot of Gold or farming effort on high-end LK enchants either.
RickJames and others hit the nail on the head...how about some item enhancement? Despite transitional gear you hope soon to replace by doing those heroic runs, still a lot can be done here.
My suggestion is to buff up with cheaper enchants, including old BC ones. For example, the +18Sta shield enchant is basically dirt-cheap now. The new +22 Sta boot enchant is also quite inexpensive. These two chants alone will give you 40 sta.
Another possibility is going back to CoT for the Tank helm buff...now this is +Def and you're already 541, but this may let you re-gem or re-itemize elsewhere for higher Sta/Armor.
Extra belt gem (crafted buckle from BS's) and crafted frosthide leg armor are good ways to get some extra Stam. These go for around 80-90g apiece on my server...not exactly piddly, but given the relatively high rate of WotLK income, not overly burdensome either. Assuming the inexpensive green-level Perfect Solid Chalcedony for add'l belt socket, there's another +76 Stamina available, even more if you splurge for the more expensive blue-level +Sta gem.
For shoulder, you can always go back to your Aldor/Scryer shoulder inscriptions.
That's the strategy I've followed...enhance my leveling & pre-heroic gear with this cheaper/older stuff, while saving up mats for the good enchants later. All in all, you can probably get yourself another 2000 hp this way for a reasonable amount of gold and little extra effort.
Another note...yes 540 is the "magic number" for defense. But as has been noted in quite a few places, that's for raiding; 83elite bosses. For heroics, you really only need 535. For 82elite bosses, you're still "def-capped" (yes, I use that term, and yes, I know it's not really a cap). Keeping this in mind, you might let your Def drop to that level if it lets you use higher Sta/Armor items.