What's causing the metallic aftertaste in my Nutraloaf?

It sounds like most of those ingredients came out of metal cans, yes? Certainly the spinach and carrots, and I imagine the beans, tomato paste, and possibly the apple sauce? Canned foods do sometimes have a slight metallic taste, especially if you don't use them all at once and continue to store them in the can after opening it: (about.com)

However, I definitely wouldn't rely on this (or any other single recipe) as your sole source of nutrition. Even if it does include every one of the hundreds (thousands?) of nutrients your body needs in reasonable ratios (which is questionable), that still doesn't mean it's good for you.

For example: if every meal you eat has lots of vitamin C and zinc, your body won't be able to absorb the trace amounts of copper it needs, since they interfere with each other. To be healthy, you need to mix up the nutrient profile of your meals.

You could use frozen vegetables instead of canned vegetables.

My guess: Lack of salt. Unless you're used to doing without, most things taste really odd without it. Anybody who's tried low-salt V8 Juice knows that bitter, metallic taste you're describing.

You might get used to going without the salt, or you might consider salting your Nutraloaf mixture some. After all, there's no reason to leave the salt out entirely if you're not making a punishment of it.

You said that you drained your canned vegetables, but did you rinse them? Sometimes just dumping out the packing liquid isn't enough. I always rinse any beans that I get out of a can at least a few times.
If you wanted to add a little taste you might want to throw in some chopped cilantro. It packs a lot of flavor and overpowers a lot of tastes. Of course some people think it tastes like dirt. I like the idea of an Uberfood. That only puts us a few short steps away from getting all of our nutrition in pill form!

You may want to carefully inspect your loaf pan. I have had times where very small scratches were causing rust to form. You do not need to throw away the pan, though. Just insert some aluminum foil before making the loaf next time. Pop it in the oven and you are good to go.

Canned spinach always has an awful, somewhat metallic flavor: spinach is very easy to overcook, and tastes terrible when overcooked. The process of canning requires high temperatures for durations that ultimately require spinach to be overcooked.

Looking at this recipe, even if you replaced the canned spinach with frozen or fresh, it would probably still turn out awful (though perhaps not quite as bad), because spinach can't survive cooking for 40 minutes. Ideally, it should cook only until it's wilted, which is only a couple of minutes at boiling temperature.

I've seen recipes that call for spinach to be overcooked, the water discarded, and the spinach to be wrung out inside a (paper or lint-free) towel to remove as much liquid as possible, before being added to something and cooked further, which I expect is to reduce the overcooked flavor.

However, I'd hardly call the result a building block for a "superfood", as the process likely discards the majority of the water-soluble nutrients, and I'm pretty sure it only reduces the overcooked flavor, so I prefer (if possible) to alter such recipes to add the spinach at the last minute, or (if not) to look for another recipe that treats its ingredients better, which is what I'd recommend in this case.