Exactly _what_ is different about being a graduate student vs. undergraduate? (Think: day-to-day)
(I am in linguistics and my master's degree was heavily research-based.)

I remember an undergrad professor of mine cautioning her class as follows: "Being a graduate student is very different from being an undergraduate student." Three months into my MA, I found myself thinking, 'no it isn't', but six months into my MA, I thought, '...okay, yeah, it totally is'. The skills that the degree aims to impart are quite different: going to classes starts out feeling similar, but over the course of any graduate degree it should evolve from 'building knowledge base' to 'reading the literature critically and coming up with new ideas'.

To me the biggest difference, psychologically speaking, aside from the big step up into the unknown (= major original research) was in terms of project scope. As an undergrad, I could run myself out of homework for the day, or the weekend, and then relax, because I'd dealt with everything I needed to do. Beyond the undergraduate level, that kind of stops happening. A grad-student in any program with a research component always has that work at least in the background, and projects are enormous in comparison to most undergrad coursework. You can't run yourself out of work for the day, or for the weekend. The research is ongoing; there's always more to be done. It's a marathon rather than a bunch of little sprints. You have to keep coming back to a thesis and picking up where you left off. At the graduate level you have to develop have a sense of how to work on something a little at a time. And how to put it away and to (try and) do something fun once in a while.