My theory is that they all have one or more of those garnet? (well, dark-reddish) reptilian eyes, let's call 'em "the H -for Hommunculi- eyes", altho' not necessarily in them ocular cavities. You must take a look at each case separatedly, let's see:
- Pride: He has them, most certainly. Lotsa 'em in the shadow. Why Selim himself doesn't have them? Maybe because he's possessed (my theory too at the beginning, now I rather think it's not the case, tho'. Dunno), maybe because since the eyes don't need to be in their place and in Pride's case they're already in the shadow, Daddy could make normal eyes for him?
- Lust: Built the usual way, nothing special about her, them eyes are where they belong.
- Greed: Same ol'.
- Envy: He has 'em, that's for certain. No matter the shape, altho' it seems he can hide them when on a human or animal one: They tend to appear on dramatic moments, specially when he's acting evil/revealing his true nature, so can't tell if he's literally capable of making them dis/appear at will, or just a dramatic plot device, with the eyes being actually always visible.
The weird thing is, in his true form he only has them in the left eye... And so many -Yuck-
- Sloth: Figures...

we haven't been able to directly see his eyes yet, always weirdly shadowed over. This is the one that intrigues me the most. What it seems clear, tho' is that he may not have right eye, as one of those red lines 'n circles is right over it. Wikipedia says it's a mask...

to them, I say.
- Gluttony: Another case like Pride... The "H eye" already being somewhere else (in this case, acting as the Doors of Truth's eye), his "regular" eyes can be different. Since I guess "realism" was not sth father was considering when creating Gluttony, he just went on with those small, "blank" ones.
- Wrath: He doesn't have them. Yes, I know what I said, but let's take a look at his eyes: his right eye is normal, human, the eye the "nameless candidate" had, and the left one has the ouroboros. Now, if we take a look at the other half-blood hommunculus ( ;p )...
- Lingreed, what do we see? Whenever Greed's speaking, if he opens one eye, it's the left. And it's our "H eye". When it's Ling speaking, it's the right one he opens, and it's Ling's.
Always: in the case he opens both, he'll be drawn far enough or with his hair conveniently over so we can't distinguish the other eye, in any case. (And it's not like he opens them that often anyway

U )
What's my theory, then? Easy: that
when a human-based hommunculus is made, one of the eyes transforms (maybe the left one always?)
while the other one (the right?)
does not, probably due precisely to their half-human nature. What I mean, then, is that Bradley would have had his eye turned into an H eye, just like Lingreed's, hadn't his ouroboros mark end up in there.
Put it another way and closing, I believe all hommunculi do have the "H eye/s", except for Wrath, just because his ouroboros ended up covering it/taking its place. With Sloth's case remaining to be seen, of course.
Whatever, I'm just bored at work

PS:
QUOTE(ehxhfdl14 @ Apr 24 2007, 01:51 AM) [snapback]534848[/snapback]
I think Sloth and Gluttony's eyes are that way because their minds are basically blank, though.
I agree with that, it's a very good, simple reason. At least design-wise, maybe not plot-wise.
EDIT: some correcshuns.