Rawr is easy to use, just load your character from armory and look at every aspect of it. Easy to find what upgrades are available for each slot. With optimizer it helps to find badly placed gems, missing or wrong enchants, suggest better talents or glyphs. Check it out. And check out your toon.
RAWR (http://www.codeplex.com/Rawr) is available for free. And with SimulationCraft (http://code.google.com/p/simulationcraft/) can be used by anyone for finding how to improve any character in World of Warcraft. While SimulationCraft is more technical (like can help you to see if it's better to stack Crit instead of Hit), Rawr is easy to use tool with direct practical recommendations.
Install it, start it, choose Load from Armory, enter details of your toon and let it download all details from WoW Armory. You will get something like following screen.
Suggestion just at beginning -check "Enforce Gem Requirements", otherwise later usage of Optimize will ignore Meta gem color requirements. If you want, you can click on "Buffs" tab and check all buffs, which are used by your toon while fighting (preferably) in raid. And in "Options" you can choose your rotation and some more aspects. For moonkin i strongly suggest to make Estimated Fight Time very low, otherwise Rawr will insist on optimization based on mana pool (known limitation). After that you are ready to use it for basic and advanced purpose. And maybe you can save the settings (File -> Save).
Right pane is something you will like a lot. At top you can switch to different aspects of your toon - basic is selected (Gear) and you can select which slot you want to see. And Rawr lists possible upgrades with DPS value of them - current is highlighted (green), so anything above is improvement. Hovering cursor over item will show you stats. And also how to get it (crafted, drop, purchaseable).
Easy to have Rawr opened in raid to see if item just available for bidding is worth of high bid or not. Or just in Heroics. Or where to got with your emblems (Frost/Triumph) or what crafted item to look for (and pay for).
I found this one very usefull mainly for alts, where i still dont remember what enchants are best. And it did help me once on main, where i used wrong enchants. Just switch to Slot:Enchant and check each slot. I will show example from my Death Knight, where Greater Assault might improve dps over Icewalker.
Stuffing right gems looks easy, but sometimes it's tricky - you have to keep Meta gem working, you want to use best gems available and you want to take advantage of bonuses whenever possible. Rawr can help - it helped me to swap gems in 4 slots of my gear to get 12 spell power more - even tho it's not really much, every bit helps. And good feeling is also something important.
Gem checking can help you mainly by finding some mistake you have made with gemming. And believe me, it's very common.
For gem check go to Tools -> Optimize. There you can use what kind of optimisation should be performed (for moonkin it generally could be sustainable dps vs. burst dps for example) and just click on Optimize button.
Look at result of optimiser by hovering cursor over item in right side and the same item on left side - you will see a GEM difference and Enchanting difference at first sight. In example it recommends changing 2 gems from STA version to STR. And changing runeforging enchant on weapon. With "Load Optimized Gear Setup" you can load changed setup to Rawr (to check talents for example).
Knowing what stats is most important for your toon (gives biggest advantage) is good. And Rawr will tell you easily by selecting from "Slot" menu. Here is example for my moonkin. It's generally correct and good enough for most of purposes. If you want to get more accurate overview of values vs. DPS comparison (for example, SimulationCraft mentioned at beginning is best choice, yet it's more technical and not just "user friendly"). Ale please be aware, this does not take caps (hit rating mainly) in consideration - you still need to know what to cap (and in which order).
Rawr does some basic calculation of possible damage output of loaded toon - for my moonkin it calculates sustained damage at 5365 dps (which is something achievable ingame, if not moving, decursing, being hit, etc). SimulatonCraft is way over this number btw, it shows something like 8000 dps - but that is based on simulation of fight with all buffs and advantages (unfortunately with zero factors that make your dps go lower)).