The only positive rules I can think of are:

A) If the item is only meant to be used by certain classes, then it must be attunable. For some reason non-attuned items aren't allowed to have that sort of restriction.

B) If a weapon can be activated, or turned on and off, perhaps with a command word or by using a charge, then it must be attuned. Because anyone not attuned wouldn't be able to activate it. This rule fails with the Wand of Magical Missiles and Dagger of Venom, among others.

However, it makes more sense to me that the default assumption is that all major items require attunement, but there are many many exceptions. My best negative rules:

1) Minor items (and some of the Common items aren't actually minor, even though some dumb editor listed them as such).

2) Expendable items that anyone can use. These are almost always minor items anyway, which might make it seem redundant. But how else to explain Gem of Brightness not being attuned?

3) Items you don't wear/wield. This includes tomes and manuals, figurines, ropes, bowls, shackles, mirrors, bags, decks, immovable rods and other tools, and vehicles like brooms and carpets. I actually find it curious that cubes, candles, and crystal balls aren't in this category, but it's believable that some DM would consider them held items.

4) Non-magical items like mithril or adamantine.

5) Simple +1 armor/weapon/shield (and +2 and +3), probably as a simple expedient so martial classes aren't screwed compared to caster classes.

5B) Weapons like Dragon Slayer, Giant Slayer, and Mace of Disruption also don't require attunement. Just speculation, but perhaps this is all so they can come as a surprise -- you thought you had a regular +1 sword all this time, and it worked like a +1 sword, but the first time you hit a dragon with it you discover (mid-battle) that it's something more. How exciting!

6) Certain individual items just get to be exceptions. Like Goggles of Night -- if the DM wants humans to be able to see, don't let the attunement rules get in the way.

I'm afraid my rules still don't cover everything. Boots of Elvenkind and Gloves of Thievery don't quite seem to fall under rule 6, it still seems odd that they aren't treated the same as other boots and gloves.

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