I will describe specifics of the language currently spoken in Zagreb, the biggest city in Croatia. This speech is also quite well known to people in the surrounding area, and they usually use it in some circumstances (they use their home dialect at home and among friends).
The language spoken in Rijeka, the third biggest city in Croatia, is similar to Zagreb in many aspects. The following features are common to both Zagreb and Rijeka:
1. Use of 'western stress'. For instance, the verb dolaziti will be stressed on the second syllable.
2. No distinction in pronunciation of č vs. ć and đ vs. dž, and no distinction in vowel length.
3. Infinitives without the final -i all the time.
4. Yes/no questions where unstressed forms are used as stressed, and simply put at the first position:
— Si ga vidio? Did you see him?
— Se bojiš? Are you afraid?
The Standard Croatian stressed form of si is jesi, while the particle se² cannot be used like that in Standard Croatian at all!
5. There are some specific words often used in both Zagreb and Rijeka, for example (the standard form is on the right):
di where :: gdje
People from Zagreb will in colloquial settings also use the following features (starting from most common):
Adverbs ending in a vowel without the final vowel. Besides examples like sad / sada, found in many places, there will be words like (Standard forms are on the right):
ak if :: ako
kak how :: kako
tak so :: tako
Specific words (some of them are common in certain other regions as well):
niš nothing :: ništa
(the rest is coming soon...)