Along the lines of what carang was saying, HKers tend to consider Mainlanders as sort of the "cousins from the backwoods." HKers consider themselves pretty sophisticated--and they are.
Most are decently educated, have traveled at least on holiday abroad and have an orderly way of going about things. They don't yell and shout at the waiter in the restaurant to get their attention (generally) but instead raise their hand and give a nod. Mainlanders do this sort of thing and it's no big deal to them. Northeasterners are pretty bad on this.
I've lived in several places in Mainland China since 2002--living with Chinese families-- and have also lived in HK for awhile--where I am part of a Chinese family. Mainlanders can be pretty uncouth. One word to describe how they behave in HK is "garish." They kind of come strolling into town loaded with cash, wearing rhinestones and shoes with fur bobbles on them (for the ladies) or mis-fitting suits, playboy belt buckles and shiny pointed black shoes (for the guys)--everything head-to-toe mismatched--which they consider to be the peak of fashion (and IT IS in Mainland China). It's easy to spot most Mainlanders in HK even if they don't say a word. Or...they are coming from Disneyland which is sort of like Mecca for them.
They speak loudly on the MTR and don't understand where their "bubble" ends and other peoples' "bubble" begins. Even though HKers get smashed up against each other on the MTR and buses, they sort of disengage and at least give off body language that tries to respect other peoples' space. Mainlanders don't. They do spit. They do scream at their kids and let their kids run wild. They don't queue up. And some Mainlanders give off a haughty attitude--like "So what? I'm rich and therefore I'm above you"--especially the ones who are only on tour here in HK or passing through--not the ones who are settling here.
And I think that last part is probably the most infuriating to HKers. Because of Mainland China's recent (last 10-15 years) economic takeoff, now there is a brand new wealthy class. HK has long been a wealthy city so the people here have developed their own ways of displaying wealth in a manner they don't consider tacky. For them to see these overnight-millionaires come in here and act like they're "just as good" as HKers is sort of like a Chinese episode of the "Beverly Hill Billies."
Now Mandarin is being really pushed in the school system so the younger generation of students at least have some interest in Mainland China and can speak Mandarin. The older generation never studied Mandarin in school when HK was under British rule so they tend to be more removed from Mainland culture.
From what I've heard in conversation a lot of people take the attitude, "Fine, come here to Hong Kong, spend your money and then leave." Yes, Hong Kong wants Mainland tourism dollars but they aren't too keen (at least the public isn't) on just breaking down the wall between Shenzhen and Hong Kong and letting it become a free for all (which seems to be the way things are headed).
I think I can see it a little bit from both sides. I have lived with Chinese families over a long period of time in Mainland China and grew to love them--even their quirks--and as it is with every country--you can never stereotype an entire group of people. My hosts are some of the most fabulous people I've ever met and I never once saw them spit in public. ;0) Now in Hong Kong, I can see how some Mainlanders act when they come here--they just are ignorant of HK customs--no one takes the time to give them a briefing on the differences between Mainland China and Hong Kong. Even if they were briefed they likely would just say, "C'mon, we're all Chinese, there is no difference" because as you may know, it is a common belief in China that if you are ethnically Chinese, though you may be born somewhere else your deepest desire is and should be to return to the Motherland even if this is not the case.