Most of the syllables in English language are weak. This is true for many other languages so I will not consider it a feature. However, this article takes a look at the general nature of weak syllables.
If you are interested in this topic then it is easy to note that a weak syllable can have only four types of peak. These are: the vowel schwa, a close front vowel in the area of /iː/ and /ɪ/ (un-rounded), a close back vowel in the area of /ʊ/ and /u/ (rounded), and a syllabic consonant.
Note: A strong syllable will have at its peak a vowel phoneme (or sometimes a triphthong) but never a schwa.
If we compare a weak syllable (that contains vowels) with a strong syllable, we can see that the vowel in the weak syllable is shorter and of lower intensity (the quality is also different).
If we look at, possibly the first word a very beginner learner of English studies, the word mother /ˈmʌðər/, we see that the second syllable, which is weak, is shorter than the first and has a vowel that can not occur in strong syllables, the schwa vowel. It is also less loud, at least according to my pronunciation.
Back to the four types of peaks, the vowel schwa is one of them and very common. If you want to learn more about schwa sound then see this article. Since it is the most common sound in English I think it shouldn’t be skipped.
The second and the third types of peaks are close front and close back vowels in the area of /iː/ and /ɪ/, and /ʊ/ and /u/. The last type is the syllabic consonant, a syllable that contains no vowels at all. These are quite vast topics so they are detailed separate articles (see INDEX page for that).