At the end of the day I don't have access to my money, or I have to call their "customer support" and spend hours explaining the problem that they created.
If the bank goes down completely (as in: goes bankrupt), my deposits are only guaranteed by the state up to a certain limit, which isn't that high.
Regulation and "liability" buy me very little in real terms.
Large banks do have bugs, and sometimes even admit it. [0][1]
The fractional reserve system basically means if there is a problem, you need to be the first at the counter. If you are not, you will only get a certain amount of money back.
[0] http://www.telegraph.co.uk/finance/2737388/Bug-crunches-Hali...
[1] http://www.independent.co.uk/news/uk/home-news/lloyds-bankin...
Edit to add: (But not quite the same as the one above)
http://www.dailymail.co.uk/news/article-2426519/Gang-arreste...
http://article.wn.com/view/2012/06/26/Lloyds_TSB_computer_gl...
In any system with deposit or investor insurance, e.g. the United States, you will get back the full insured amount (up to $250 000 for bank deposits, $500 000 for investor funds). Beyond that you will be treated as the senior creditor that, as a depositor or investor, you are. If you want more certainty as to the return of your capital, buy Treasuries.
I probably should have mentioned I am in the UK. You get back up to £170k here ($280k) but I guess my point still stands :-)
Edit: (Oh, that's for a married couple by the way. £85k if you're single.)