Here is a list of some of the reasons there are a million people in Kyiv protesting the official government election results: (see the OSCE preliminary findings for a good, balanced discussion of the irregularities)

• 96.3% turnout in Donetsk, 88.4% turnout in Luhansk (they didn't even get this high a turnout in Stalinist times)
• Almost 5% of voters in this election were added to voter lists on election day, and almost all of the additions were using Absentee ballot forms. (meaning people coming in from other areas, and thus not necessarily having residences in the places they voted) This is expected to have been a way to allow voters to be transported to multiple election locations to vote.
• Widespread allegations of employees forced to take out Absentee Ballots and turn them over to their superiors at work, thus depriving them of the ability to vote.
• Hundreds of local voting commission members from the opposition in pro-Yanukovych areas were dismissed from the commissions just before election day
• Hundreds more opposition observers trying to monitor polling stations were kicked out of those stations during the vote count.

In addition to the election-day fraud, protesters also had the following activity from the first round of the election and the campaign to get their blood boiling:

• Voting lists that were inexplicably riddled with errors during the first round. As many as 10% of voters were turned back at polling places because 1) their names were misspelled, 2) their birthdates were wrong, 3) their patronymics were wrong, 4) women still had their maiden names, and so on. Encouragingly, this problem was mostly corrected in the second round but the massive disenfranchisement in the first round was aggravating.
• The campaign was marked by ugly slander, completely devoid of facts or honest discourse, the majority of which was on the part of the administration. By taking advantage of "administrative resources" (the ability of current government officials to use their positions to conduct campaign activities) this kind of slander could be frequently trotted in front of voters, like these examples from a public train right before the first round.
• All major television media, and most print media, which are owned by pro-administration businessmen or by the state, were absurdly biased in their reporting. The OSCE found that 43% of the material covered was covered in a "conspicuously similar manner". What this indicated to the OSCE was that it was likely that opposition claims about temnyky are correct. The temnyky are allegedly small briefings from government officials telling reporters what and what not to report, as well as how to report. [later addition of supporting evidence: journalists from top TV stations 1+1, Inter, and public station UT1 have publicly renounced the "biased reporting methods" they said they were told to use, some in front of the crowds at the rallies] In any case, the OSCE also found that state channel UT1 gave 64% of it's political and election primetime to Yanukovych, 99% of which was positive or neutral reporting. In contrast, Yushchenko got 21% of political and election primetime, of which only 46% was positive or neutral.  This was typical.
• In September, Yushchenko disappeared from his campaign as he was rushed to a hospital in Vienna. He emerged from the hospital horribly scarred. He still bears the swollen and misshapen face he returned to Ukraine with a couple weeks later. The opposition thinks he was poisoned, and no doctor has found the agent used on him yet.  The government initially made pacifying gestures, but eventually came right out and said he'd faked it, then decided that he hadn't faked it, it had be an allergic reaction to botox injections due to complications arising from long-term alcoholism.
• Every single pro-opposition media outlet in the country (and every neutral outlet, like Radio Liberty) was either shut down, or subject to allegations of terrorism, or accused of tax fraud, or accused of holding an improper license... To take Radio Liberty as an example, the radio stations carrying its programs decided that it didn't make sense to broadcast them, even though Radio Liberty showed that the business was profitable. Radio Liberty then negotiated with another station, and during negotiations, the man negotiating with them died either in a car accident or in a mugging, I can't remember which it was attributed to.