What do you wish you knew as a student before you became a researcher?
Research can be great, but I learned it can be so hard.

First, Google "should I do a phd" and then "should I do a PhD in ". There are tons of people who succeeded and failed in academia and writing about it, about what's it going to be in Academia, and so on. Two examples:


Nature's special on the point of a PhD
You and your research


Beyond that, here's a few things I've learned:


Being a researcher is a career job, not so unlike being a lawyer, a manager.


After all, a paper has to change the scientific knowledge on the
topic. You're going to teach respected scientists something they
didn't know and something they thought was wrong.
I hear that in life sciences, the career aspect can extend to people sabotaging
fellow researchers (that is, competitors).

Being more intelligent does not (necessarily) mean being a better researcher. If you're admitted to a decent program, you're more than smart enough for research. Managing your work (in all senses) can be more important, even though supervisors can help you with (part of) this in the beginning. But it's also about discipline and character:


Choosing a good question to work on (supervisors can help) - the question should matter (or you should make it matter, if you can).
Keeping an overview of what you're doing is important: otherwise, you might find an excellent solution to a problem which in the end is the wrong one.
Working enough on it (keeping focus among distractions of grad school can be hard).
How do you choose which way to approach a problem? Some approaches might not ever work, but others will only work if you keep at them long enough. So you need to be tenacious, and be able to change mind, depending on the moment. You need to delude yourself that you'll succeed until you do.

A good supervisor is extremely important. Nowadays it's extremely hard to start doing research "by yourself". On the one hand, there's too much written knowledge to navigate (literature). On the other hand, there are often unspoken assumptions which you won't learn from reviewers. It helps if he's available enough for you.
A supervisor must also fit you, character-wise. You mustn't be friends, but you need to function together, and neither you nor your supervisor study how to occupy your position.
Communication is extremely important. You're supposed to learn something and then tell your peers (and the world). And nowadays, if your peers don't get it, it's your problem (because your paper gets rejected). Hence, learning technical writing was very important for me.


Of course this is what I learned, which is influenced by my weak points, my experience and my field — I'm saying this because I'm sure somebody will disagree.
I'm a programming language researcher, so I invent and design stuff (programming language). And evaluating design is hard. So much that many ideas are not right or wrong per se - you can make it right or wrong by determined work (see also this). But other disciplines probably experience similar phenomena in different form.