Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
The major difference between a soup and a sauce is the amount of water content. I checked out your blog entry on The Great Tomato Massacre to get a better idea of what was happening.
In each of the cases, you were on the right basic track. I've noticed with Vegetarian Cooking for Everyone that there are the occasional step that I disagree with, or might add in, and they occasionally seem a bit optimistic with their time estimates. So let's start with your first batch, where you cooked the tomatoes whole to break them down, ran them through the food mill, then cooked some more.
Instead of breaking down the tomatoes whole, I would suggest cutting them in half and seeding them first. This will save your trouble in the food mill, as you won't have nearly as many seeds to contend with. If I knew a way to do the same with concord grapes, fresh grape juice wouldn't be nearly as dangerous as it is. Although that may be why it tastes so good.
The second part of this recipe that will help is to let it reduce a lot more. A whole lot more. Leave it simmering for a few hours to get it to the right consistency if you need to. That will pep up the tomato sauce tremendously.
For the second recipe, I think the biggest problem was that you weren't using the tongs, and you didn't put the tomatoes in cool or ice water before you tried to peel them. That combination would have saved you some grief. But the real problem is pretty much the same as the first recipe: time. Because you have so many tomatoes, there's a lot more volume there. If you're using a tall cooking vessel, you don't have very much surface area for the water to break free through, so it takes longer to reduce.
I think you would have had the best results if you'd have used the roasted tomatoes from your third batch and turned that into a sauce. I know you'll likely do that in the future, so keep in mind that they should be great. The roasting process took out all of the water early on, which concentrates the flavor. There's probably even some maillard reaction going on there that will help out the flavor as well.
Why is this not a problem with canned tomatoes? Well, a couple of reasons. The first may be if you drain the water from the can and just use the tomatoes. The second may be that you're a little more patient for the rest of the cooking if you've not had to do a bunch of work on the tomatoes already. The third is that the tomatoes, when canned, have some acid added to them for the canning process. That will add a little extra spike that you're missing.
Mostly, though, it's all about the water. Reduce that sauce down and you'll have something much more like the tomato sauce that you're looking for.