1 - Do you want to 'be a writer' or do you want to write? If you find yourself saying that you want to 'be a writer' but haven't actually written much, while making excuses to yourself and your friends - I don't have the time, I don't have the space, I don't have a computer - then you may need to have a rethink.
2 - Getting an agent doesn't mean that you'll get a publishing deal. Even if you win the query letter lottery, ten to one you'll be asked to rewrite. Many times. And even when you've done those rewrites, you still may not get a publishing deal. At some point, you may realise that you've wasted the time you should have spent writing another book on rehashing the previous book.
3 - Very few people make a living out of their writing, and this situation has only got worse. Are you prepared to diversify and do other things to earn actual money? The average 'advance' - if you get one, which is debatable - is tiny. £1000 or £500 is not unusual for a book that has taken a year or two to research and write. Lots of jam tomorrow in the publishing world.
4 - If you have written fiction, you'll be told that nobody is reading fiction. If you have written non-fiction, nobody is buying that either. If you write popular fiction, it's not literary enough. If you write literary fiction, it's not popular enough. I was once told that my work was 'too literary to be popular but too popular to be literary.' It's the equivalent of the indrawn breath when you are trying to sell a car. Nobody wants that particular model, although they absolutely do want the model they are trying to sell you. The Long Tail seems to be an unknown concept. (Read the book. It's a revelation.)
5 - For most small to medium sized publishers, editing is not what it was. A good editor will ask all the right questions and in answering them you will make the book better. But most are now freelance, and many publishers simply can't afford them.
6 - Are you prepared to do almost all publicity and promotion yourself? You will be expected to contact libraries and local venues. You will probably have to organise your book launch yourself, and many bookstores won't be keen to host you unless you can guarantee sales. All of this genuinely (as opposed to point 1, above) eats into good writing time. And yet you feel compelled to do it for fear of missing out on sales.
7 - The physical quality of your precious baby - aka the book - may not be nearly as good as it was even a few years ago. In fact it may involve paper like Soviet Era Polish bog roll. (Of which I have some experience.)
8 - Once you get a modicum of traction, with a reasonably popular subject, you may be asked to talk about your book. Many commercial organisations will expect you to do this for nothing. My maddest moment was realising that I had hauled myself across the country, done a ten minute talk for a big chain bookstore, (there were four other participants I didn't know about and a tiny audience) paid for an overnight stay in a horrible room, and trekked back home. All at my own expense.
9 - There are, of course, some organisations that may invite you to speak, give you a good meal and somewhere nice to stay and buy copies of your book as well. It has happened to me and I was incredibly grateful. But it's rare and getting rarer all the time.
10 - Given all of the above, you may want to consider going it alone. You won't be alone. You can get help along the way. But it isn't a simple solution either. These are shark infested waters, and you need to be careful and committed. Nevertheless, it strikes me that the reverted and new titles I'm now publishing myself under my own imprint are good quality. Good physical quality as well, with beautiful covers, and nice paper. I'm the same writer I ever was, albeit with many more years of experience. And you know what? When it comes to the reading public, that long, long tail of people who like books, nobody cares who published them.
They never even notice.