It is really simple to judge a piece of gear as long as it is a simple piece of gear. That is all I plan to show here, a simple guide to help show those that might not know how to evaluate if a piece of gear is really an upgrade.
Please read the tier and trinket section below for exceptions.
For this post we will be comparing gear that does not offer any additional bonuses such as tier gear or trinkets. Just a simple comparisons to help you decide on which piece is better and if an item level upgrade is really an upgrade.
How do you judge if a piece of gear is an upgrade?
1) Find your stat weights:
Go to google, if you do not already have your favorite class forum, blog, or resource bookmarked, and search for your spec stat weights. Usually typing something like "Marksman hunter 6.2 stat weights" will be enough to find them. You are not looking for a list of which is better, so you might need to click a few links. You are looking for a list with numbers.
When I looked for marksman I came up with these results, thanks goes to Azortharion and Niix for their awesome hunter guide that supplied me with these numbers.
2.80 - Weapon DPS
1.00 - Agility
0.90 - Attack Power
0.71 - Crit to 53%
0.64 - Multistrike
0.60 - Versatility
0.59 - Mastery
0.58 - Haste
2) Use those stat weights to get a value for the item:
For this example I am going to use my necklace, the darkflight necklace from BRF heroic and compare it to another neck I recently received from the HRC raid on normal.
We will need to strip the stats off the pieces to compare them, then add the value of the pieces up into one total value, which will be the value we place on the item to compare the two.
Heroic Darkflight Necklace.
Warforged, with a Gem Slot. (Item level 691)
166 agility x 1.00 = 166.00
111 critical strike x 0.71 = 78.81
111 multistrike x 0.64 = 71.04
75 critical (from slot) x 0.71 = 53.25
Total value of that necklace= 369.10
Normal Chain of the Lidless Eyes
No enhancements (Item level 700)
180 agility x 1.00 = 180.00
166 haste x 0.58 = 96.28
74 mastery x 0.59 = 43.66
Total value of that necklace = 319.64
As you can see, the 691 item is actually a considerably better item for me. This comes mostly from the fact I have a 75 stat gem in the gem slot, but even if I had a 30 or a 50 gem in that gem slot it would still be better. As long as the gem slot is not empty, the lower item level piece, in this case, is much better.
Note: Many times items with gem slots are where these cases of the lower item being better than the higher item level come into play and these items are the ones I suggest you always check to make sure if the upgrade is actually and upgrade.
3) Keep the higher item level item:
Even if the higher item level item is not an upgrade for you in your current spec it is an item level upgrade. If you plan to pug the only way people have to judge you, if they do not armory, is to use your item level. When you sign up for a group your item level that they see is your best item level, including those items in your bags.
So even if you get an item level upgrade that is not a performance upgrade, you should hold on to it if you plan to pug. While you will be gearing smart wearing the better piece even if it is a lower item level, you having the higher item level shows up when you apply to a group and it could help you get accepted into pugs. Once you are in the pug you will be judged based on your performance, so do the best you can do, abide by mechanics, and no one will ever know you actually have a lower item level.
The 6 Item Level Tip:
Without going into detailed break downs of every item for every class I will make a blanket statement that is more often then not true unless your spec has any stats that are dreadfully bad for them, as in under 0.40. In which case the number could be higher than 6, but 6 seems to be the average, at least for hunters.
Usually anything more than 6 item levels higher is an upgrade for most classes, regardless of stats, if there are no gem slots involved.
Using the above example, even if it is a 9 item level difference, with the 691 neck with perfect secondary stats vs a 700 item level neck with my worst secondaries you will see, even if I take away the gem slot, that the 691 item level neck would be a 315.85 vs the 700 item neck's 319.64, making the 700 item an extremely minor upgrade, but an upgrade none the less, at 9 item levels. This will, in most cases, be the case.
The 6 item level tip is a basic guideline and not to be taken as law even it will be true more often than not. It works well for hunters as their stats are very close to one another whereas some classes do not have that benefit and have larger gaps between best and worst secondary stats. If your stat gap is larger, the number would be higher.
The 6 item level tip is for the lazy person, like me with alts. If the item you are wearing has your two best stats and the other item is only 6 item levels higher and has neither of them, do not switch. If it is 7, switch. Either way, I'd still suggest checking it out using the easy formula shown above to make sure if you even care a little but about getting the maximum potential out of your gear, but with the lazy method, you will never hurt yourself too much either way.
Tier Bonus and Trinkets:
You can not, and should not, ever use what I said above to judge tier sets and trinkets. The reason for this is that the tier set bonuses could, for some classes, greatly outweigh the stats on a piece of gear that would seem like a large increase but break a tier set bonus in the process.
For example, the T17 hunter bonus for BM hunters is still wonderful if you have not snagged the T18 set yet. And with the increase of bestial wrath to 15 seconds up from 10 seconds in an upcoming patch, it makes it even better.
So even if you have 4 670 pieces for the tier set it would be much better than replacing it with a couple of 700 items you got out of a mythic bag and two 695 empowered apexis pieces you made on Tanaan. Even if you would see a substantial item level increase from equipping those 4 pieces and a fairly decent upgrade from their base stats alone, having those extra stats does not (in many cases) make up for breaking what is a very good tier bonus.
I suggest you read your classes forums somewhere like elitist jerks, mmoc, or the battle net forums to see if and when you should break a tier set. Do not use the above mentioned method to judge in this case.
Or, and I believe this would best, you could learn to use simcraft and play around with it some to help you decide if breaking the tier set will work for you. There is no simple way to decide like I showed here. Even a super tier set will eventually be beaten out by raw stats alone, if the gap gets great enough.
The same goes for trinkets. Never use the base stats on trinkets to judge a trinket. Trinkets often can be deceiving, in terms of item level. Such as the trinket off the world boss with the tertiary stats on it. Sure it is an item level of 705, but a 630 trinket from a heroic dungeon would, in many cases, be a better trinket than that one.
For trinkets I suggest you look at the previously mentioned sites, or your favorite bloggers suggestions, instead of using this formula or judging based on the face stats alone.
I hoped that helped some people learn how to judge which piece of gear is better for them. It is not always about item level, but it is always about stats. Whichever piece has the best stats for you, regardless of item level, is the best piece for you.