What qualities does Hong Kong Mandarin have? With Mandarin making more and more in-roads into Cantonese speaking Hong Kong, I'm wondering what characteristics set it apart from Standard Mandarin or other regional Mandarin dialects and/or accents.

I'm not asking so much about native Mandarin speakers that have relocated to Hong Kong. But about Hong Kongers whose first language is Cantonese but also speak fluent Mandarin.

When a Mandarin speaker visits Hong Kong what stands out most?

Other areas where Mandarin is spoken with identifiable regional differences have Wikipedia articles, but Hong Kong does not. Compare the articles on Mandarin in Singapore and Taiwan for instance.

(If it is felt that this question would fit in better at linguistics.SE I am not opposed to migrating it there.)
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 (Aside: this helps to explain the outdated Western spellings of &quot;Beijing&quot; as &quot;Peking&quot; and &quot;Chongqing&quot; as &quot;Chungking&quot;--the qs and js were originally velar!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.