Conventions, the expectations you should share with your reader about communication, are arbitrary. They are sometimes formally agreed on; sometimes they're unstated. In both cases, you are usually still expected to follow the conventions. Ignorance of law isn't an excuse.
Conventions are meant to make writers' and/or readers' lives easier, not harder. Sometimes the reader's interests dominate which conventions are adopted and followed; less often, those of the writer dominate. Like any kind of semi-binding arbitrary agreement, conventions change to meet new circumstances. Discourse communities which don't change die off.
For some, conventions change too fast. Bring up grammar as a topic, and you'll hear complaints about the speed or fact of change in the conventions governing language usage. For others, change can't happen soon enough. For instance, do you know the usage rules behind how to use "whom"? If you do, you are among the minority. Chances are, "whom" is on the way out of the language and the conventions of polite, formal language usage. It's headed the way of "thou" and "thee." "Y'all" or/and "you guys," on the other hand, that is, a different second person plural from "you," are on the way in. It's up in the air which one of these words will be arbitrarily decided on as "correct."
In addition to surface level issues like grammar, usage, and the format of a particular kind of text, there are conventions which govern deep level concerns, such as, what constitutes legitimate evidence, who can say what, the genres in which a conversation will occur, the content and diction which are allowed, and how meaning is constructed.
Conventions are idiosyncratic to individual discourse communities, but remember, there are no firm borders in human behavior or socities--especially in something as basic as communication. Discourse communities interact and interpenatrate, and there are frequent disagreements about which conventions are right and which are wrong. Those in power usually decide.
In the author/audience/text rhetorical triangle, there are conventions of how interpretation takes place as well as in the form of the text. Encode a text wrongly, that is, don't follow the conventions, and you'll add to the noise in the communication circuit and reduce the chance your audience will understand you fully and your chance to fully realize your purposes in communication.
Think of writing conventions as mind fields you *have* to cross, and the analogy will pretty much hold. Be careful to follow someone else when you enter into a discourse community and are discovering where and what the conventions are. Move carefully until you know where the conventions are. Step off the known path with some trepidation. Most of the time you'll get out intact, even if you've stepped off the path, but the chance is there for some bad consequences. Armor helps. Armor in communication is knowing the language and conventions of those in power in any particular rhetorical situation. When in doubt, keep quiet. If you still have to move prior to learning what conventions are in place, follow your experience and try to armor yourself in the conventions of those in power in the larger community.
Learning the conventions expected of you in each rhetorical situation in which you seek to communicate is essential to producing successful texts, but no one can know them all. There are too many discourse communities, conventions, and each is always in flux. More importantly, conventions are always embedded in the relationship between author and audience; in fact, conventions are specific to each rhtorical situation. You can, however, learn most conventions by talking with existing members of your discourse community, by looking at model texts and using them as a touchstone, or by doing research. As always, practice and repetation helps.
I encourage you, for instance, to research the conventions surrounding the use of portfolios as a genre in the academy. This information will prove valuable in this class, in putting together your senior portfolio, and in keeping the portfolio of your work you might use to get a job in industry. In this class, the portfolio is relatively low stakes. Your senior portfolio and the one you might use in your career on the other hand...
As always, write with questions. You might, as an additional forum to the Q&A threaded discussion on the class site, emails to me and one another, responses and exchanges in your docs, also comment on and ask questions about the posts in the blog in the blog itself. I left the comments turned on to facilitate just such communication.