In no particular order (many of these reasons are linked.)
Acting like Bush was an awesome president who made few mistakes:
This of course is a popular one with Neo-Cons and RINOs who want to pick up where we were: Bush may have been more decent and sincere as a person than portrayed, but he made many huge mistakes. One of the biggest was not acting to limit The Fed pumping and housing policies that helped fuel the housing bubble.
The best, simple answer is to make people aware these policies were fueled by high ranking Democrats and liberal interest groups and ask what Barack Obama did to received $126,349 in contributions from Fannie Mae & Freddie Mac?Telling people who mostly knew better that Bush handed Obama a healthy economy was never a good idea. The right plan is to say, Obama inherited grim the results of policies he often supported.
Not fighting fear of budget cuts by telling the truth about the vast debt and entitlement bomb.
Again we have a problem with Neo-Cons and half hearted Republicans either afraid to confront the welfare state or afraid to touch the "third rail" of politics by telling a public the truth about the entitlement state. Surprise- A lot of people are already pretty aware that something is wrong- making light of it by acting like Romney could easily keep Medicare and Social Security as they are made him look shifty.Rick Perry hit the nail on the head- Social Security is a Ponzi Scheme-- and it's high time the GOP stoked anger at this rip off and passed the bulk of the blame for it where it belongs. Of course that may mean throwing some RINOs under the bus, which sounds good to me. The massive shift in recent years towards the GOP by older voters tells one, the welfare state is far less universally loved than most imagine.
Allowing Democrats to portray themselves as defenders of the poor rather than existing power elites; crony capitalists; too big too fail banks and the speculative casino economy.
I attended only one early Tea Party rally on April 15 and strongly remember the big theme was anger at bank bailouts, The FED, Wall Street insiders and crony capitalists shoving their mistakes on the taxpayer --" the aristocracy of pull", in Ayn Rand's words. Sadly, the later campaign and the 47% remark made it look like a "crude rich vs moocher poor" conflict that played into Obama's hands. Many, of the hard working poor know they are not moochers and many "moochers" long to be independent citizens.
Not acting as defenders and passionate supporters of the grass roots economy and the hard working, deserving poor
This relates closely to the GOP's dramatic, continuing fall among Hispanic and Asian immigrants. Instead of actively pointing to the ways Democrats have undermined opportunity among the poor- The party refused to mention the word poor.
Not actively promoting a pro Legal immigrant agenda
It didn't take a genius to figure out the party was counting on low growth, Rust Belt, white voters - a strategy that rightly seemed to appeal to a backward nativist base. The GOP must send a clear message that it supports an open clear path to the American Dream for legal immigrants-- and become their active champion.
Being Anti Gay Marriage.
Not sure what else to say about that. Here in Pittsburgh, you couldn't get to first base about Romney- without hearing homo-phobe. I actually know a Romney voter who felt safer showing his opposition to Obama by having a Gary Johnson yard sign for that reason.
Literally, never engaging urban voters
This is a huge subject worthy of a later post. The term "disrespect" fits perfectly, It takes nerve to expect a group of voters you almost never engage directly to vote for you. Shunning large diverse urban areas while scouring exurbs and rural areas fed the meme that the GOP is a racist party, with an agenda they are trying to hide from urban voters. Of course- Obama did almost the exact same thing in reverse- the main difference being that cities are a growth voting block.
The reality is that a candidate has a limited chance to fertilise and till new ground in a short campaign and to a large extent has to deal with the existing perceptions of voters. The GOP's immigrant and urban problem is a result of years of neglect. The positive thing is we are doing as well as we are.