The essence of mathematics is not to make simple things complicated, but to make complicated things simple. ~ Stanley Gudder
So, scale factors, what the devil are they? These are the relative value of each stat; in the same way apples, oranges and bananas don't cost the same as each other, not all stats are equally good at increasing your DPS. Spirit is nice to have and gives a decent damage boost but not as good as spell power. Hit typically gives the highest DPS gain of any stat but only until you reach the cap and then it's worthless. You can pick up many opinions on what are the respective values of each stat on any number of forums and websites but none of them are tailored to you -- the scale factors you just generated are.
If, for example, your scale factors showed that spell power had a value of 2 and crit 1 then SP is twice as important to increasing your DPS as crit. You would need to find an item that increased your crit by 20 to be as valuable to you as one that increased your SP by 10. Hit can be somewhat misleading in these things though. By default SimulationCraft considers the DPS gain from a gain in each stat except hit, where it works the value out by considering the DPS loss from losing some hit. This way you can consider swapping out some hit items or gems for SP, haste etc. and know if it's worth doing. You can get the app to not treat hit this way by selecting to "use Positive Deltas Only" in the scaling tab of the options.
If you have reached the hit cap then gaining more is worthless but losing some of what you have could be a bad move. This can be hard to judge with scale factors alone and this is where having SimulationCraft plot a graph of DPS gains for each stat is frankly, awesome. Call me a bookish nerd if you like but I love the DPS scaling chart, it just makes the whole issue of stat importance so simple.
DPS Gains chart
If you find this chart a little daunting take a little time to look at it. DPS is shown vertically, so the higher up the chart the better the DPS. Left to right is either an increase or decrease in each stat with your current value in the middle. So the chart starts on the left with showing what your DPS would be if you lost 200 points of each stat -- not all of them together, just which ever one you are looking at. So if you look at the purple spell power line and see where it is furthest left, that gives an idea of what your DPS would be if you lost 200 points of SP.
If you look at the chart at the top of the page you can see that most of the stat lines are fairly straight. The red one for hit however rises more steeply than all the others and then flattens off. The point at which it flattens is the hit cap -- the point at which gaining more hit (moving right on the chart) doesn't increase DPS (moving up on the chart). You can see that this cap is about 50 points below their current level. Also in that example chart you can see that the steepest line on it is the spell power one - both for gains and losses of the stat. This means that this character would gain the most by gaining SP and lose the most by losing it. Very handy to know when choosing gear upgrades.
One final little tip for helping with gear choices. You may have noticed that links were provided to Wowhead and Lootrank. These links are based on your scale factors and will show a list of gear for each slot and the total value of each item. Both these sites do the job very well, I like Wowhead for it's filtering and tabbing of the slots and I think the way Lootrank allows you to baseline your own gear is fantastic. I would strongly recommend people try them both and see what they offer. Personally I use Lootrank as they attempt to include proc bonuses for trinkets and weapons but you have to be careful here as it assumes they will always proc to the maximum they can, which isn't going to be the case for those that trigger from "your healing spells". Neither of these sites will include set bonuses in their scoring either but don't let that put you off, just factor it into your consideration when reviewing them. Don't forget you can try out upgrades in the Wowhead profiler and then import them into SimulationCraft to try them out -- SimulationCraft does factor in set bonuses.
SimulationCraft offers many more tools and toys to play with and really takes the strain out of number crunching. As Blizzard introduce more proc-based mechanics into the game it is tools like this that will have a better chance of modeling them than static calculators and spreadsheets. It's certainly not a perfect tool and it's definitely not going to magic you into a better player. What it does though is make analysis a whole lot quicker, easier and more measurable.
Blood Pact is a weekly column detailing DoTs, demons, and all the dastardly deeds done by Warlocks. If you're curious about what's new with Locks since the last patch, check out WoW.com's guide to patch 3.3 or find out what's upcoming in Cataclysm from the BlizzCon 2009: Class Discussion Panel.