I would suggest that one of the primary factors limiting the semantic fusion of compound nouns in English is the inherent flexibility of the basic English word.

Buffalo buffalo buffalo Buffalo buffalo buffalo

In the wake of the creolization of Old English due to sustained contact with Scandinavian invaders from the North and Norman invaders from the South, English lost the vast majority of its inflection, leaving only a nearly-agglutinative genitive case, a pitiful set of, at maximum, four verbal inflections, an oblique case hanging on to the pronouns by its fingernails, and a handful of irregular verbs and nouns.

Thus, for a wide range of basic English words, it's impossible to tell whether the word is an adjective, a verb, a noun, or perhaps an adverb by looking at it in isolation. Homonyms abound, making English more similar to Japanese or Maori in this aspect than to any other Indo-European language. All that can resolve the ambiguity are articles (probably why English hasn't lost them despite the loss of noun gender), the infinitive marker 'to,' or the addition of certain endings like 'ing' or 'ly.'

English, a 'Germanic' language

Since there are a lot of words in English whose ambiguity you simply can't resolve, these 'nouns' can't form compounds, even if they're clearly nouns in meaning. So, in the Swedish example of 'mobiltelefon' vs. 'mobil telefon,' 'mobil' as an adjective could conceivably take the ending -a if it was modifying multiple ambulatory telephones: 'mobila telefoner.' This applies yet moreso in German, with its bewildering array of genders and strong/weak adjective endings.

Even in Dutch, a Germanic language tantalizingly close to English, an adjective must take either a null or a schwa ending, and one can build grammatically correct, if not exactly sensible compound nouns that look like a sick joke to an English speaker (de randjongerenhangplekkenbeleidsambtenarensalarisbesprekingsafspraken). Thus, splitting 'mobil' from the noun in Swedish has meaningful utility--it provides the opportunity to inflect 'mobil' and firmly establish it as an adjective, with the associated change of meaning.

In contrast, there's no utility in differentiating mobiletelephone from mobile telephone in English. The only possible adjective inflections--comparative and superlative endings--are impossible to apply to a host of adjectives including 'mobile' (more mobile, most mobile--not mobiler, mobilest). 'Mobile' occupies a hazy limbo between noun and adjective no matter what you do to the noun compound as a whole, evidenced by the use of 'mobile' as a perfectly normal adjective in such phrases as 'mobile strikeforce,' but the additional presence of "a mobile," or "my mobile" as a clipped form of 'mobile telephone.' You might even be able to make a verb of 'mobile,' though this would smack of business management jargon and be redundant to the verb 'mobilize' already in common use.

To fuse, or not to fuse?

You'll notice that I wrote 'strikeforce' instead of 'strike force' in the above example. Both are acceptable depending on the style guide you adhere to, though 'strike force' is certainly more common. The difference between 'strike' and 'mobile' that allows strike to be compounded is that 'strike' can only be a noun or a verb. Since there's no ambiguity, English speakers can freely revert to the Germanic tendency toward compound nouns (or stick with the separating space if they're feeling more Latinate).

This 'rule of ambiguity' also explains the contrasting example 'cellphone'-- it's not primarily because 'cell' is a shorter word. 'Light,' for example, is just as short phonetically, but 'lightfixture' just doesn't look right at all, even though 'light' in this case is absolutely meant to be a noun. 'Cell,' however, can never be used as an adjective--it already has a fixed adjective alternative in the form of 'cellular.' Since there's no ambiguity, English speakers are free to write either 'cell phone' or 'cellphone.' 'Cellularphone,' however, is right out.

The rule of ambiguity also explains why compounds of more than two nouns are exceedingly rare. The longer the compound, the more unstable the construction becomes. One might say 'putdownworthy' in conversation, but in written language at least one hyphen would be necessary. Might 'downworthy' be a noun, and would it then be only a noun? For that matter, is 'worthy' really an adjective? What if one was speaking of a vast category of books, 'the putdownworthy,' that a reader may safely ignore? Fusing two nouns is a dangerous business, a risk that some people simply don't take no matter how unambiguous the two words that might be squashed together. Fusing three nouns invites ruin (or at least misunderstanding).

Put 'putdown' down as putdownably putdownable

Interestingly, the case of 'putdown' actually has nothing to do with semantic fusion or compound nouns, although it is related to these phenomena. 'Putdown' itself is an example of an unambiguous compound: the verbal phrase 'put down' could never function as an adjective in isolation, so one is free to write either 'put down' or 'putdown' as its (very recently) nominalized form. Given this inflexibly verb-derived noun, if it is to be used as an adjective, one must give it a specifically adjectival suffix: '-able.' The prefix 'un-' then negates the entire new adjective, yielding your final product 'unputdownable.'

Now, personally, I would never use this word. It strikes me as offputtingly awkward and it could either mean that the book cannot be physically put down, or that the book cannot be insulted. Just one too many derivations away from the original verb 'put.' But this is a tad hypocritical of a man who just used the neologism 'offputtingly' only two sentences ago!

Ugh.. this is giving me a headache. Head ache? Head-ache? UGH!

In the end, compounding in English is still an area of the language fraught with idiosyncracy and irregularity. My 'rule of ambiguity' is at most only a guideline. For example, I inserted a hyphen into the compound 'verb-derived.' I have no idea why, though perhaps it had something to do with the adverb 'inflexibly' that preceded it. Likewise, no one uses 'styleguide' even though neither of those words has the ambiguity of being both noun and adjective.

The more flexibility inherent to a word, the longer a word, or the more derivational endings a word already has attached to it, the less likely a word may be fused in a compound. The safest way to play it is to refrain from fusing compounds under any circumstance, though this leaves something to be desired stylistically. It is a matter perhaps best left to intuition for the native speaker, and brute force memorization for the foreign speaker.