Some have more ridges than others, some are decorated by jewelry, some are plain. Some have no ridges. What is the significance of Klingon brow ridges?
Foreheads as a mark of heritage
The shape of Klingon brow ridges is a strong sign of one's heritage. This is particularly clear when one studies Worf, his brother Kurn, his part-human son Alexander and his grandfather Worf; their foreheads all have a distinctive ridge going down the center.
One can also study members of the House of Duras, who all have a central bulge with an array of indentations, surrounded by an almost wreath-like crown.
Likewise, compare B'Elanna Torres' "full Klingon" forehead with that of her mother: Link
There is also a recorded incidence of a Klingon having surgery to masquerade as the member of another house. After the operation, his ridges had been radically modified to resemble those of his father. Link (DS9 SPOILER warning)
Several regional Klingon words for "forehead" (the standard word for which is Quch) also suggest a link to ancestry. One such word is no''och, which - if naively translated using 24th century Klingon - means something like "ancestral tunnel". There is also tuqvol, which appears to include the word tuq, meaning "house" or "dynasty".
Adornment as a reflection of culture
Given that Klingon views on family vary greatly (from Klag's "A Klingon is his work, not his family." to Worf's "One is always of his tribe."), it makes sense that there would be different norms as to whether and how one should adorn them.
Role in sexual selection
The shape of the ridges may also serve some role in sexual selection; perhaps some ridge patterns are generally considered more attractive than others. They may also serve to prevent inbreeding, much as body odor is believed to do among humans; a Klingon may find persons with ridges similar to his/her own less sexually attractive.
Foreheads as a status symbol
One might speculate that the introduction of the Klingon augment virus - which, among other things, made Klingons' foreheads smooth - would have made Klingons even more aware of their ridges. After all, before said virus, forehead ridges could easily be taken for granted. Indeed, the Klingon word vIl can mean either "be ridged" or "something that is just there".
Many novels suggest that such is indeed the case: Klingons with forehead ridges were very much the upper class of Klingon society, and there were several incidents where Klingons artificially modified their foreheads to improve their standing. Those with ridges were called HemQuch, meaning "proud forehead", and those without were called QuchHa', which could mean either "misforeheaded" or "unhappy". (I've been liberal in my translations here)
And, of course, there is that oh so famous Klingon insult: Hab SoSlI' Quch! ("Your mother has a smooth forehead!")
Speculative link to patriarchy
The members of the House of Duras that we have seen all have extremely similar foreheads. For instance, compare Duras (anno 2153) with Toral (anno 2372). Though separated by 200 years, their ridges are almost identical.
Now, one probably shouldn't read too much into that; it's probable that they had their prostheses sculpted from the same mask (either to save money or to make the connection more clear to viewers). However, if one wishes to take things a bit too seriously (and clearly I do), one might wonder if this has some connection to dominant and recessive genes, or genetic linkage.
Specifically, could it be that many features of forehead shape are Y-linked? That is, that your forehead will likely resemble that of your father more than that of your mother? In that case, if the Duras we saw in the 22nd century is a direct patrilineal ancestor of the Toral we saw in the 24th century (as is commonly believed), the similarity of their foreheads wouldn't be as surprising.
Note, however, that forehead shape can not be determined solely by your father's genes: Miral, B'Elanna Torres and Miral Paris make up a matrilineal triple, and both B'Elanna Torres and Miral Paris had human fathers, but they nevertheless share similar forehead patterns.
Nevertheless, if there is a pronounced patrilineal element to forehead shape, it might be part of an explanation to the strong patriarchy in Klingon society. Perhaps they reason that because this clear sign of heritage is most prominent in male heirs, it is male heirs that should champion one's house.