I assume, based solely on my experience with American politics over the last twenty years that there must have been a number of states that could have gone for the democrats, even if a Republican blowout was inevitable. What were the major errors or missteps in the Mondale campaign? Or, conversely, what were Reagan's specific tactical successes?
There were many factors, first was that many Democrats assumed after the recession in '82 (unemployment was over 10% at it's peak) and the assumption that Carter lost due to his poor skills and Anderson taking many votes that that all they had to do was show up. But the economy improved a lot by '84, and since the USSR boycotted the Olympics in '84 to pay back our boycott in '80, the US won a huge number of medals in Los Angles. A further problem was the democratic party was seriously divided (at that point their were still many 'southern democrats' that soon would become republicans). Mondale was a compromise candidate, and while picking a woman VP gave him a small boost, he was not a engaging speaker or campaigner. Lastly the Republicians ran a brilliant campaign, with the first use of 'modern' visuals.
David Broder provides a good summary here http://archives.chicagotribune.com/1984/10/19/page/27/article/the-election-is-in-reagans-hands
There have been times when certain states were safe for one party or another in American history besides the present, but this hasn't been the rule and it was not the case in 1984. 1920, 1928, 1932, 1936, 1956, 1964, 1972, all saw very large electoral college margins for the winning candidate. Farmers and extractive industry workers in Western states still gave significant vote shares to the Democrats, urban professionals were still quite Republican, there were still a lot of white Southern Democrats but not so many that the South was at all "Solid" the way it had once been, lots of other ways the party coalitions were different than they are now and one consequence of that was that there weren't very many states like Utah or Vermont that consistently gave overwhelming majorities of their support to one party or another, so scoring an electoral college win like Reagan's, Nixon's, or LBJ's didn't require the outlandishly massive popular vote victory you might imagine. Party loyalty was also much lower than it is now with more voters who would switch from one party to another; when these voters all went the same way as they did in 1984 it could produce popular vote margins that are hard to imagine today. It was a very different political world where electoral college blowouts could and did happen a lot.
Now, why did this one particular one happen? The simple answer is that the economy was growing really fast in the months leading up to the election and people, consciously or subconsciously, decided to reward the incumbent. This happened because the Federal Reserve under Paul Volcker raised interest rates to unprecedented heights between 1979 and 1982, causing the worst recession since the Great Depression but also ending the period of stagflation that taken up most of the '70s, when, contrary to the predictions of Keynesian economists, inflation and unemployment were both elevated. This recession was not prolonged like the Great Depression or the 2007 recession and the economy rapidly returned to trend, with a sharp recovery featuring high real growth and low inflation, a combination not seen since the '60s. Anyone lucky enough to be in the office at the time would have done similarly well.