Regarding #3, the foursquare article presents itself as an analytic analysis of data. It has good and bad points. I think the intro is weak but it seems favorable in comparison to the article you shared.
1st sentence from 4square: "With over 400 million check-ins in the last year, it’s safe to say that our servers log a lot of data." It seems they will use millions of unique server records (their data) for analysis.
1st sentence from UncommonGoods article: "The holiday season is rapidly approaching, and as gift-giving authorities, we’re doing our part to stay informed of recent gifting trends." It seems somehow there is a connection between gifting trends and the angriest cities in America, and that relationship will be further explained in the article.
I am not a fan of either intro, but the 4square one seems more relevant. The 4 square article doesn't actually care which cities in America are the angriest. That is a means to an ends. Their article is about how data is captured and analyzed. They offer details as to their queries are designed and how Amazon's S3 service and Ruby on Rails can be leveraged to analyze data.
As you read through the comments on that article, notice it reads more like a Reddit post. The comments are mostly off-topic and random rather then related to the article. This indicates to me the respondents are not all that interested in the article.
IF your article was related closer to the 4square article, and the commenters were actually interested in the topic, then every commenter would be a potential member of your audience. You could certainly leave a comment in the 4square article and share it was the source of inspiration for your article. You could then reach out to any commenters as well and make them aware of your article.
With respect to research, the Distilled link James shared is exceptional. I find myself visiting the Distilled site more and more recently as excellent articles such as that one are shared.
As for choosing the next topic, the world is open to you. If you want a more engaging topic concerning "angry cities" some recent topics are:
These topics have readers so completely engaged they are willing to get up from their pc and take action. People are literally in the streets, turning over cars, and to use a relatively calm word, interested. You can pick a completely different topic but find something readers have a strong want or need to read.