As you can see in the title the words "weg" and "Weg" confused me somehow. Is the only difference between them the upper-/lowercase writing?
In written language, yes, unless you start your sentence with "weg" as in
But in spoken language, the "e" in "Weg" is longer.
In terms of phonetics:
Weg, [veːk], close-mid, frontal, unrounded vowel
weg, [vɛk], open-mid, frontal, unrounded vowel
The comparative "lighter"
was attributed to the e in Weg by Emmanuel. [e] is technically closer to [i] than [ɛ] is, so if you perceive be [i] to be "lighter" than cat [æ] for instance, then I guess you could say so.
Although an example of a longer [ɛ], like Käse [ɛ:], shows us how length is a bigger factor here.
English examples for [e/ɛ] are hard to find. Maybe men for [ɛ] and a Scottish/Irish pronunciation of days for [e].
As mentioned before, "weg" and "Weg" are pronounced differently:
Nonetheless, both words share a common etymology. The reason for the difference in pronunciation is the following:
In Middle High German, "weg/wec" [vɛk] had a short vowel and was used both for the adverb and the noun. When declining the noun, there was for example the plural "wege" ['vɛ.gə], also with a short vowel. In the transition from Middle High German to New High German, vowels in open syllables (no consonant at the end) were lengthened; i.e. one has the plural "wege" ['veː.gə] but still has the adverb and the singular noun "weg" [vɛk]. In New High German, the singular noun "weg" changed its pronunciation, per analogy to the long-voweled plural, from [vɛk] to [veːk]. It was not recognized by New High German speakers that the adverb "weg" once belonged to the same paradigm; thus it did not get the long vowel.
Upper-/Lowercase is not the only difference between the two, they also differ in pronounciation: "weg" has a short E, whereas "Weg" is spoken with a long vowel.