Many voters in Franklin County, Ohio, home to the city of of Columbus, experienced waits of five hours or more on election day. Lawsuits against the Ohio Secretary of state were filed that day on behalf of some of these voters. There were many reports of voters leaving before they could vote, in some cases because they were too old or infirm to be able to wait in line for hours.
Voting machines in Franklin county had to handle 170 voters each, on average, much more than the 86 voters per machine in Summit County (Akron) and 68 per machine in Cuyahoga County (Cleveland). [LINK]
The conventional wisdom is that "record turnout" swamped the precincts and caused the lines. This seems to be false. There was high turnout throughout the rest of Ohio on November 2nd, but not in Franklin County.
In fact Franklin's 60.95% turnout rate was LESS than in 2000, when the rate was 61.30%. However voter registration in Franklin county had increased by 24% since 2000. You would expect nearly all of the new voters to go to the polls, so turnout should have been much greater. That seems to indicate that many thousands of voters didn't get a chance to vote in Franklin, perhaps because of intentional vote suppression tactics by the Republicans.
Statewide voter turnout was about 70% in Ohio this year, up from about 64% in 2000. In Hamilton County, home of Cincinnati, where vote suppression tactics don't seem to have been as intense, turnout was 73%.
If Franklin county had 73% turnout this year, an additional 76,115 votes would have been recorded. Let's say these voters would go for Kerry 75% to 25% -- a reasonable assumption given that Democratic-leaning precincts seem to have been targeted. That means a net gain of 38,057 votes for Kerry.
38,057 votes isn't enough to overcome the highly-publicized gap of 136,000 votes that Kerry needs to win the state back from Bush. But combined with the provisional vote count yet to be completed, losses from vote suppression tactics in other counties like Cuyahoga, and future adjustments due to fraud or machine malfunctions [LINK], it could have been significant.
It appears that a lack of voting machines in certain precincts was the cause of some long lines. According to the Columbus Dispatch, "many polling places in inner-city neighborhoods had fewer voting machines than during the last presidential election." [LINK]