Colloquial language in research papers
I've come to associate complex sentence structure with "correct" scientific reporting, and anything less complex as too informal/colloquial


For science to be correct it has to pass the review process. The reviewers are usually experienced and native to the narrow field that is covered in the paper, so however complex the language, they can handle it and gauge the science behind (unless we are talking about extremely obfuscated writing, of course). From that standpoint, the language complexity doesn't matter. 

The degree of colloquialism that is found in papers usually depends on the field. As an example, when computer science began to shift towards personal narrative ("we did...", "our research..." vs. "if x is done...", "one can deduce...") a number of years (or decades) back, the new papers gradually became more colloquial. Compare that to a typical mathematical or physical paper.

You should also consider that there are a lot of non-native English speakers out there, who struggle to express their thought in a foreign language.

Finally, inexperienced scientists and students tend to "overdo" the writing, using complex structures to convey maturity and confidence. Judging importance of information and expressing it eloquently are skills that are acquired well beyond grad school.


  Is there room for "writing like you talk" in journal publications?


Sure there is, as long as it is professional, i.e. avoid jargon, jokes, etc.


  How do you figure out the correct balance?


That comes with experience, every subsequent paper you write will be an improvement. It is important for you to strive to become better. Experience in writing is not the only thing that will benefit you, but experience while reading as well. The more papers you read, the more patterns of good and bad style you will encounter. Reflecting that on your own style will improve it.