Pros
Amazon will teach you a lot. You'll be around brilliant people all day and are bound to learn something new from them, even if you pick it up by osmosis. The things you do are likely to ship. Not everything does; as with every company, some projects get canned or put on hold indefinitely, but most things move pretty quickly. Even if your particular project never sees the light of day, it's likely that some of the things you did will get repurposed. You'll have a pretty good idea of where you fit into the machine based on your job title and the people around you. This isn't the sort of place where you're going it alone and everyone is siloed off; there are literally thousands of people who can answer questions or collaborate. If you meet or exceed "the bar", you will be recognised to a degree. In most cases, effort in equals reward out. Being so process-heavy can actually be a blessing in disguise as resources are always available for any problems you might run into and things are fairly well-documented. Amazon gets things done and one of the key values is having a backbone -- you're expected to stand your ground if you disagree. This lends itself extremely well to things like eliminating code debt and keeping things moving; there's no groupthink. It can sometimes become a little bit adversarial, but it's not a big issue and is a small price to pay for the advantage it gives the organisation. For a huge company, it's outright nimble. Everything is data-driven, which I'll also mention in the Cons section. As a pro, though, if you have the data to back it up, you'll be able to get what you want. Additionally, your performance is measured with data-driven metrics, so your review shouldn't come as a surprise -- good or bad.
Cons
I hope you took pictures of your work-life balance while you had it, because it's gone the second you step in the door. If you're not in the office until 2 in the morning some nights, you're probably not meeting the bar. This is an unfortunate fact. Salary is average, perks are nil, and bureaucracy is outrageous (though you have to expect the latter in a company of 89,000 people). "The bar" is either something you stand on or it hangs over your head. Not "raising the bar" means you're toast eventually. Amazon has a monumental turnover rate; most people will give up after a year. Office politics happen, largely in the form of needing to suck up to the right people in order to be heard and having to manipulate colleagues to get them on your side. Again, fairly common in an organisation this big. Everything is data-driven. You can't go on instinct or gut feeling; if you don't have the data, you don't have an argument. Your experience means nothing if you don't have data to back it up. If something comes down from Jeff, people drop everything in order to kowtow -- which would be fine if it wasn't completely inconsistent with Amazon's stated values. You might lose your job for daring to argue with the decisions of a hypercompetent genius from Mars, but if you want to be consistent with having backbone, that's a chance you have to take. The fabled "question mark emails" mean someone's not sleeping for the next two weeks.