What's the deal with ilevels?

So... I'm confused. This is my natural state, so normally I'd just blink and hit my head on something, but I figured I might ask.

What's the difference between ilevels?

Take Deflection Band - Item - World of Warcraft and Keystone Great-Ring - Item - World of Warcraft.

Both are epics, at ilevel 200. Both should logically be roughly similar, right? A bit here, a bit there. More block, less stamina, etc. And yet when you compare them - Item Comparison Tool - World of Warcraft

Is the armor and dodge really worth that much that it equals out with the defense and hit?

Do ilevels really mean much? And is there a difference between a 200 epic and a 200 blue? Should they be similarly itemised?(I'm colourblind, so I often don't know what's epic and what's not. Bring on 3.1's colour blind options!)

I'm just trying to understand how it works, how to work out the relative value of something, if there is such a thing.

Basicly higher itemlvl equals higher stats.
If same itemlevel, epic items has higher stats than blues, blues has more stats than greens.


Theese two ring are an example.
Runed Ring - Item - World of Warcraft blue itemlevel50
Onyx Ring - Item - World of Warcraft green itemlevel50

The blue rings "random" stats are 9/9 and the green gets 7-8/7-8.
You also notice that if the "item points" are spent on a single stat it gets higher, but not by much (Green of stamina = 12stam & green ring of boar gets 7-8 stam/str).
The blue ring also has one additional stat, spellpower, where the green only has the "of the XXX".

This is basicly how it works, each stat has a cost, you can get one "high" stat at the same cost of two a lower stats.

In TBC the cost of stamina went down, ie "ring of stamina" has higher stamina than "ring of strength" has strength.

Im sure something has changed with WOTLK, but im not sure what.

iLevels are basically a 'budget' for buying stats on an item. Therefore, an item with higher iLevel can have more stats and higher amounts of each stat. The formula for the budget calculation is pretty complicated, but the basic idea is that it costs more to have 10 of one stat than it does to have 5 of two stats.

So the relative value of an item is based on two things: its iLevel, and its stat allocation. Going back to the formula, it's possible to have 2 items, one which has stats like:
25 strength
50 stamina
20 crit rating

But if they itemise it better, you can have an item with the same iLevel but stats like:
25 str
30 stamina
15 crit rating
20 armor penetration