Spreads that get wider and hit everyones stops - Ive had stops hit when the underlying share price didnt even come close.
Delayed/refused orders - Can be annoying particularly in a fast moving market, seconds can make the difference between profit and loss.
Credit accounts - I would avoid these like the plague unless you really no what you are doing. IG tried to convince me to take out a credit account when they knew I had never spreadbet before (Which is extremely dodgy) they phoned me up a few times to ask when I was going to make my first deposit.
It is very easy to lose a lot of money quickly if you dont know what you are doing. When I started I was placing £5-£10 a point bets, which was stupid. I made money very quickly in the first week, but I had to pop out and I had a set too large a stop on a dow trade. It went against me by 70 points and wiped out 60% of my capital.
Spreadbet companies are out to make as much money as they can, they are a business and make money either by you losing or by hedging. Unlike a broker who make money from you commission and it is in their interest for you to make lots of money.