As many bloggers talk about a lot, the tenure track, dream job, pyramid scheme of academia is NOT all that starry-eyed grad school applicants chalk it up to be. This is a highly competitive, high-pressure, intensive and sometimes very unfair world. It can be soul-destroying to try to crack into, and only a small percentage of people who want to try it get the opportunity. It's not NECESSARILY a happy place where pottering about on what you love will result in an unassailable job for life with summers off and a four day week. If you're extremely lucky or shrewd, you can turn it into that, but a positive trajectory on the tenure track is NOT a given for ANYONE.
I got myself into this job knowing exactly what the risks and benefits were. I might not have known exactly how I would DO the job, or exactly what the daily function of a TT assistant professor would entail, but I certainly harbored no illusions about what would be expected of me. I also knew full well that it was MY RESPONSIBILITY to figure these things out, that nobody was going to hold my hand and make sure I kept up. I am expected to know what level of grants/publications/teaching excellence will be considered 'enough,' even without any documentation or policy or handbook or helper to describe it in any detail. The job description is intentionally vague: "they," the department and the university, don't want to be too specific for a number of reasons. A lot of tenured faculty resent the idea of getting bean-counted, so they don't want someone devising metrics by which to check off their productivity and progress--and if they made something like that for the TT'ers, then they themselves might have to be compared to see where they measure up. More importantly, the university doesn't want to put itself in a position where denying someone tenure can be tracked back to positive metrics and result in a lawsuit.
So, yeah--it sucks that you never quite know what is expected of you and nobody is going to tell you one way or another if you are measuring up or not. Until tenure, you are an at-will employee, which can be a precarious situation. You can get kicked off the track before you even get up for tenure through non-renewal of contract: at a lot of places (maybe all?) if your trajectory is not promising enough by year 3, your bosses can decide not to renew your contract beyond your 4th year--meaning you never even get a chance to go up against the tenure and promotion committee. It's a luxury to land in a department with high quality mentorship, colleagues who actively participate in helping you succeed from day one, and a department head who does his/her best to guide you frankly and pragmatically and/or who gives you USEFUL feedback at years 1 and 2 that helps make sure you don't get stuck in a non-renewal situation your 3rd year. (For the record, I am fully aware of how lucky I am to have this luxury! My environment is awesome!)
But on the other hand, getting a notice of non-renewal of contract in your 3rd year means you still have a whole year before your contract runs out. A whole year! You can keep doing what you've been doing, wrap things up, apply for other jobs, and all the while keep getting paid according to your original contract. How many other jobs give you that kind of leeway? Most industry jobs have at-will contracts, and when they decide to terminate your position or lay you off for whatever reason, you have about 30 minutes to clean out your desk and head home. Normally there's some severance involved, but only if you are being terminated for some reason beyond your control. Not performing up to expectations (which at least are usually bean-counted and documented a little more thoroughly in industry and government vs. academia), I am guessing, would not be considered 'beyond your control,' so you usually do not get severance etc. Also, while some benevolent companies provide mentorship and support for their employees to work their way up the system, it's certainly no MORE common than in academic departments and I'd wager a guess that it is LESS common.
So at the end of the day, tenure track assistant professors actually have it pretty good. Even if we have to get $1,000,000,000 in grants and 250 papers in 5 years all while living in our offices, eating out of vending machines and climbing uphill in the snow with no shoes on every day... we still have the sole responsibility and opportunity to control our own trajectory for success as best we can, and not many other people in the world can say that.