1. Publishing a book ie producing something you can hold in your hands is the easy part. You just pay a printer, typesetter, cover designer etc and they do the work for you. Companies such as Lulu.com effectively do the same, but get their money from a slice of the cover price.
2. Distribution is the problem. It's very hard to get into bookshops that either buy centrally or buy from wholesalers ie most of them. That's not to say it can't be done, but it requires hard work.
3. The paperwork can be tricky. Ebooks are easier, but you'll still need to keep receipts, send out invoices etc. Self publishing is running a business, even if it's only got one product - your book.
4. It is much much easier to make a success from non-fiction than fiction. Non-fiction means you can target a defined market. I published careers books; I sold them to careers officers at secondary schools. Fiction sells to...people who like reading stories?
5. Not all your friends will buy a copy, and neither will all their friends. Despite reading about success stories the chances are you will lose money on self publishing. At best you will break even. Sad but true. At least if you epublish you won't have 2,457 books stored under your bed.
6. Books are heavy. 2,457 books under the bed will strain your joists. I worked out that 500 of my books were the same weight as a baby elephant. No wonder the car died after carting a small herd around.
7. Self published books usually look amateurish (cartoon covers or illustrations by your partner/neighbour/child are a giveaway). It is worth getting them professionally designed. Ditto professionally edited.
8. Book marketing and publicity is a full time job and buying in expertise is expensive. That's why niche books for small markets work.
9. Discounts are high in the book business. 65% is not unusual for the chains plus you'll have to pay the p&p. And then wait for 30+ days to get your money. If you sell directly to customers then you keep more of the cash, but single copy orders eat time and energy.
10. Define what you want to get out of it. Make lots of money? Hold your book in your hands? See it on the shelves at Waterstones? Win the Booker? Work out what YOU really really want to get from this, and make that your target.
I loved self publishing and as a mum with a baby and a toddler, living in the middle of nowhere, it was the only way I could make some money. I averaged about £10,000 per annum from it and my children grew up knowing how to stuff envelopes with mail-shots and stick stamps on parcels (fun for all the family). I stopped when I realised I was spending most of my time managing others and hardly any of it writing. So I wound the business up and gave myself two years to get a novel published. But that's another story.