It will depend on the law level of the world around the port. A "lawless" world will have plenty of vice and danger to go around, and the port will be a haven for relatively honest interstellar trade. A high law world will have relatively few outlets for vice and danger, and the startown will often end up harboring those outlets.
In between, and present in nearly all cases where the world is somewhat developed, you're going to see a lot of trans-shipping facilities. Warehousing, import/export, transport hubs, and those secondary businesses that benefit from proximity, including brokers both corporate and freelance. Where the potentially seedy side sneaks in is when these associated businesses are not fully automated, and their employees need lunch, an after-shift drink, or a crash pad close to the office. Older Startowns will probably have gone through a number of cycles of getting seedy, getting downright dangerous, and getting cleaned up. That doesn't even take into account the occasional natural disaster. Having your onsite hydrogen fuel stores spring a leak and go "rain bomb" on everything within two miles is "natural", right?
And remember, some ports in the central Imperium are thousands of years old and will have gone through half a dozen institutional changes in legal status with regard to the worlds they are on and the stellar polity above them.