Lead... If the hose doesn't say it's safe for drinking water then the parts used tp make the hose may contain lead, and also other rubber off flavors. I picked one up at wallmart for $15. Beyer safe than sorry.
Also, a lot of people have suggested that you need to limit the flow to one gallon per minute to prevent "breakthrough" where some particles can slip through the filter due to force. So pulling 5 gallons should take 5 mins, I use a on/off valve to limit my rate. Built one of the whole house filter setups, cost aprox $75 to build.