Much of the following answer comes from this 1999 study, as well as Cheng (2009), and some of my own experiences.
Let's first get the usual suspects that identify Cantonese-accented Mandarin as a southern accent out of the way:
- lack of retroflex consonants. This is a given, merging them into their alveolar (and not palatal) counterparts. Hypercorrection is also possible, but in my experience rather rare in strongly HK-accented Mandarin.
- merger of /n/ and /l/; which in casual Cantonese (the so-called 懶音) itself tends towards greater frequency of [l]. Hypercorrection is common too, and the intermediate nasalised lateral [l̃] is also common across many southern regions.
- a glottal [h] as a realisation of /x/ (Pinyin: h-). This I think is under-researched, and I don't know whether there are many isoglosses that have been drawn for it.
Now for consonant differences which help to distinguish Cantonese-accented Mandarin from other southern Mandarin and southern-accented Mandarin varieties:
- distinction of /f/ and /x/ (Pinyin f- and h-): merger of these two is not a feature of Cantonese-accented Mandarin, but there is certainly a different assignment of /f/ and /h/ (and null!) in Cantonese, and this is frequently transferred over. 福州 and 湖州 are generally distinct, but 呼 will have [f] rather than [h~x] and 護照 will have the null consonant instead of [h~x].
- preference for [kʰw] over [hw]: this is a kind of phonetic adaptation towards Mandarin, where the "standard" pronunciation [xw] is perceived closer to [kʰw] than to [hw] (which would be the 'regular' adaptation). This comes from Cantonese not permitting /h/ in front of /u/ (all of those in Middle Chinese turned into /f/ instead in Cantonese).
- voiced retroflex fricative /ʐ/ (Pinyin r-) merging with [j] instead of [l] or [z]: I believe this is distinctive to Cantonese, but it is a marker of a strong accent. The merger into [l], more common across the other southern regions, is/was almost as common in Hong Kong.
- depalatalisation of the palatal series (Pinyin j-, q-, x- becoming z-, c-, s-): this is again toward the extreme side, and is also related to the vowels.
- monophthongisation of diphthongs, both rising and falling. This affects so many sounds, and is particularly pervasive. Monophthongisation is nonetheless also found in many southwestern areas (Sichuan and Yunnan Mandarin, looking at you!), as well as in parts of the rural north. I don't know if there are any monophthongisations that are particularly Cantonese: but loss of medial glides in e.g. Pinyin nian, qiang, quan, luo is very prevalent. Loss of final [ʊ] in ou is also reported.
- The central vowel /ɨ/ merging into [i] in a strong accent, or [œ] or [ɵ] in less pronounced ones. This Mandarin vowel is famously elusive anyway, but the fact that it is mapped to /i/ is a strong indicator of an accent from the southeastern non-Mandarin speaking regions. In less strong accents, it often retains some fronting and some rounding, presumably because it is phonetically close to Cantonese [œ].
- The Pinyin e vowel, which is mid-high in the standard /ɤ/, is centralised to [ɘ].
- Other Cantonese vowels affecting Mandarin are well documented, but come from straight-up transfer from Cantonese. There are certain "colourations" of the vowel that remain in the vowel, e.g. in Pinyin ang being slightly rounded, but I haven't seen many studies yet.
The strongest impression of the accent comes from the supersegmental features, in my opinion:
- Mandarin third tone (low falling-rising tone in isolation, low tone on its own, mid rising tone in sandhi) maps to Cantonese fifth tone (low rising) consistently. Interestingly, this is an example of correspondences in history (Mandarin third tone corresponding either to Cantonese second tone or fifth tone, both being derived in large part from Middle Chinese "rising tone" 上聲) and phonetics leading to slightly different but noticeable outcomes.
- Mandarin first tone (high level tone) is often lowered to a mid tone. This seems to be related to distinguishing Mandarin first tone from fourth tone (high falling), whereas in modern Cantonese, high level and high falling are allotones of the one phonemic tone, first tone. Ironically, the historical correspondences mean Mandarin first tone corresponds to Cantonese first tone, and Mandarin fourth tone corresponds to Cantonese third (mid level) or sixth tone (low level), for the majority of lexemes, which makes the switch a lot harder. This lower first tone in Mandarin is also a feature of Taiwanese Mandarin.
- Singaporean Mandarin, Taiwanese Mandarin, and Cantonese itself are all more syllable-timed than Beijing Mandarin (which is then more syllable-timed than HK English or even Italian); and I have a strong impression that Cantonese-accented Mandarin falls into a similar range. This also contributes to neutral tone being ignored.
- What might be different is the fact that Cantonese does have phonemic length, but its effect on Cantonese-accented Mandarin has not been explored much.
The rise of the 港台腔 "Hong Kong - Taiwan accent" was probably more to do with the presence in the media of those particular accents of Southern Mandarin than with any "one real accent", and its association with the petite bourgeoisie and their lifestyle.
In terms of grammar, the use of Cantonese sentence-final particles is also particularly salient in HK. The use of HK-related vocabulary (e.g. 打的 for "to call / take a taxi") would further serve to distinguish HK Mandarin from Guangdong Mandarin as a whole.
There is now a Chinese Wikipedia page on Cantonese-accented Mandarin. However, it definitely needs a lot of work!