What qualities does Hong Kong Mandarin have?
I am hardly an expert on this topic. I know basically nothing about Cantonese-influenced Mandarin per se, but I'll offer an answer of the variety that I think hippietrail is looking for. Hopefully other people will be like &quot;I now understand what a good answer to this question is supposed to look like, and furthermore, I know more than that idiot Stumpy Joe Pete! I'll answer it!&quot;.
As I mentioned in an answer to a similar question, most southern Mandarin has the characteristic of merging zh ch sh into the z c s series (e.g., &quot;zhi&quot; pronounced as &quot;zi&quot;). Cantonese does this, so I expect HK Mandarin accents to do so.
In Cantonese (among the many many other differences in pronunciation) the g and k are preserved in places where, in Mandarin, they have been palatalized into j and q. For example, 北京 is pronounced &quot;bak1 ging1&quot;. (Aside: this helps to explain the outdated Western spellings of &quot;Beijing&quot; as &quot;Peking&quot; and &quot;Chongqing&quot; as &quot;Chungking&quot;--the qs and js were originally velar! As for the p/b k/g thing... blame the Europeans.) Sometimes this shows through in people's Mandarin. For instance, some HK classmates of mine mistakenly guessed the pronunciation of 季节 as &quot;gui4jie2&quot; (instead of &quot;ji4jie2&quot;).
It's not only ks and gs that became j and q. There were also fricatives and affricates that palatalized in similar circumstances (before [i]/[j]). See here and look for the /tɕ/ /tɕʰ/ /ɕ/ parts.
Possibly related to that: I once had a very young student with a rather hilarious Mandarin accent (I don't know if she was a Canto speaker). In at least some circumstances, zhi would become ji, chi would become qi, and shi would become xi. This culminated in her yelling about a spider:

老师，老师，有蜘蛛啊！
Standard: lao3shi1, lao3shi1, you3 zhi1zhu1 a!
Her: lao3xi1, lao3xi1, you3 ji1zhu1 a!

I say that it's possibly related, because in Cantonese, 师 = si1 and 蜘 = zi1 (where the i's are actual [i]s! Not like Mandarin!). By analogy to Cantonese words pronounced sik and zik, you would expect the Mandarin to be xi and ji. So, like I said, possibly related.