Ancient Greek had a feature not present in Modern Greek known as phonemic vowel length. Vowels and dipthongs could be either long or short, and this was not always written. Long vowels would be pronounced for longer, probably somewhere between 1.3x to 2x the amount of time as a short vowel. Some words could be distinguished from another solely based on vowel length. An example of this phenomenon is ἵσταμεν vs ῑ̔́σταμεν, which differ in tense based solely on the length of the initial ι.
Of the Greek letters and dipthongs, ω and η were always long, as were ει, ου, υι, αυ, and ευ. ε and ο were always short. α, ι, υ, αι, and οι could be long or short depending on the word. Some greek texts will distinguish long α, ι, and υ by writing a macron over top of them, like ᾱ, ῑ, ῡ. αι and οι are usually long, except in certain noun and verb endings. There is usually no marker for if they are long or short.
Greek also at one point had what were called long dipthongs. These were ωι, ηι, ᾱι, ηυ, and ᾱυ. ωι, ηι, ᾱι would later come to be written with the ι underneath the main letter since it wasn't pronounced by that point. The long dipthongs would likely have had a long initial vowel followed by a glide.
Greek also had so-called geminated consonants. Any time a ρ began a word, or two of the same letter were adjacent, the sounds were held longer. For ρ, this meant a using a trill instead of a tap. For stops, this meant pausing momentarily before releasing the stop. For fricatives this meant simply producing the fricative for longer.
Not all the systems listed on this website use this feature. This feature had started to weaken in some dialects sometime in the 1st century A.D., and disappeared for most Greek speakers sometime between the middle of the 2nd century A.D. and the Byzantine era.