Will the election yield a clear winner or another messy coalition? How will smaller, special-interest parties affect the vote? And is it possible Britain will have to go through another election all over again before Christmas, as the current deputy prime minister warned earlier this week?
With so many questions looming, here are the issues likely to shape today's result and its aftermath:
1. A Scottish surge
The rampant rise of the Scottish National Party has become one of the biggest stories of the election. A recent poll found that the SNP could win every single one of Scotland's 59 parliamentary seats, the majority of them currently held by Labour.
If the expected landslide comes to fruition, the SNP's influence will dramatically increase in the House of Commons, where it currently holds just six seats.
The SNP is Britain's version of the Parti Quebecois. It campaigned for Scotland to break away from the rest of the U.K. in September's independence referendum.
Even though it lost, the SNP has benefited from the way the referendum energized Scottish voters who want to wrest more power away from faraway London.
The prospect of a nationalist party wielding a disproportionate amount of power at Westminister has worried the other party leaders who face the possibility of having to work with the SNP if the election doesn't produce a clear winner.
2. The party of "fruitcakes, loonies and closet racists"
Nearly 10 years ago, current British Prime Minister David Cameron was asked what he thought of the United Kingdom Independence Party. Not mincing words, Cameron dismissed it, saying it was made up of a "bunch of fruitcakes, loonies and closet racists."
Fast forward to today and far-right UKIP has become a force in British politics, having won last May's U.K. elections for the European parliament with 28 percent of the vote. It was the first time in more than a century that a party other than the Conservatives or Labour won a national poll.
UKIP's main objectives — Britain's withdrawal from the European Union and tighter controls on immigration to the U.K. — resonate with voters who feel "left behind" by the traditional parties, according to Dr. Matthew Goodwin, author of Revolt on the Right: Explaining Support for the Radical Right in Britain.
However, its members are gaffe-prone, with several having been expelled from the party for making outwardly racist comments.
Even its charismatic leader, Nigel Farage — whose influence comes in part from his image as the everyman, down at the pub — is facing an uphill battle to win his constituency of South Thanet.
Still, UKIP's power might not lie in the number of seats it wins but rather how the seeming popularity of some of its positions—particularly its stance on Europe—has prompted a shift to the right by the other parties.
Several commentators have labelled Cameron's pledge to hold a referendum on Britain's membership in the EU if he holds onto power as an attempt to win back Conservative support from UKIP.
UKIP also threatens to take away votes from the main parties in hotly-contested ridings, contributing to the near impossibility that any party will win an outright majority.
3. 'Milifandom' and the online campaign
One of the more entertaining aspects of this election has been the internet's often zany reaction.
Take the unlikely rise of Milifandom, for example. It's an army of smitten, mainly teenage, girls who've been swooning over Labour leader Ed Miliband on social media.
Their posts range from moody selfies captioned, "Thinking about Ed Miliband," to discussions over how Miliband made cameo appearances in their dreams.
Some even like to superimpose his head on iconic movie characters like Rambo and Superman.
Milifandom is just one example of how this election has gone viral.
Another includes the involvement of comedian/activist Russell Brand who has been opining on the election on his YouTube channel. (His interview with Miliband has been viewed more than 1.2 million times.)
It's hard to say what impact any of this will have at the polls, but with youth voter registration on the rise in the UK, a positive online perception among youth could translate into votes.
4. Possibility of a hung parliament
It is extremely likely that Brits will wake up to a hung parliament the day after the election, meaning that no party won an absolute majority.
If that happens, the country could be facing weeks of political horse-trading as parties try to form a coalition government.
"We are likely to see a very messy outcome: a multi-party outcome which could involve three, possibly four, parties taking control of the country." said Goodwin.
The first test for the new government would come in early June, just less than a month after the election, when parliament will vote on the government's plans outlined in the speech from the throne.
If the vote goes against the government, it would mean more political uncertainty for the country.
5. Push for electoral reform?
The long-term impact of this too-close-to-call election might be more serious debates over electoral reform in the U.K., says Goodwin.
The first-past-the-post system, the same electoral format used in Canada, failed to result in a clear winner in the 2010 elections and it's expected to do the same this time around.
Critics also say the system doesn't allow the House of Commons to accurately reflect the percentage of votes each party received.
For example, the SNP is expected to win less than five per cent of the national vote but more than 40 parliamentary seats. UKIP, on the other hand, is on track to garner around 13 per cent of the vote but only a handful of seats.
"That's a real challenge to the legitimacy of our two-party system," Goodwin said.