The biggest thing might be Industrial Spending. If that's not set to 100% you will lose. End of story.

If it's not that, I don't necessarily know. I'm playing on Normal (I think, I can't remember what the difficulty was) and I'm dominating. AI civs call me up and give me money for being so cool.

The game has a learning curve, I'll grant that. However, you can learn how to win. You have to play somewhat aggressively at first, and I don't mean militarily. That initial 5000 BC is meant to be spent. If more than 500 BC remains in your treasury by the end of turn 20 or so, you aren't spending it fast enough. You have to buy colony ships (don't wait for them to be built); scout worlds with your flagship and build a scout or two to assist. Buy up factories (at least the first one) on each world you colonize. Remember that EVERY world needs at least 2 factories to actually get stuff done.

Follow the above advice too, especially about minor civs. Also, remember to TRADE. I usually ignore weapons for a while and go right into trade. For most of the game, your tax won't be enough to let you run your industry at 100% and still turn a profit each turn so trade is essential. Thus, forego weapon tech early and get trade ASAP. Then build freighters. I haven't found galactic resources to be as key as others, but economic starbases can really help your worlds in all aspects.

Oh, you also do need some military, or the AI will find you to be a tasty snack and declare war.

Yes... that is rather annoying. I find its best to chose a race to attack, and take over their planets as fast as possible, hopefully getting them to surrender their last couple to you. Just a strategy that totally helped me in my last game.

I also almost never buy ships. In the beginning I always set spending to %100 then I'll buy the first factory (or two) on my home world and the first planets that I settle. Then I generally let the planets produce things on their own. Even just doing that will drain your money pretty quick which is why it's important to find other civilizations and trade with them. In the beginning I prefer trading techs for techs rather than money but later in the game (during that first war phase) I'll just go for money and do research on my own.

The hardest part of the game is the part after that initial colony rush. In my view you only have a year or so to build up some base techs, get your economy rolling (again, at this point you will be nearly out of money), and then start building a war machine to challenge another race. You'll have to sell techs to get money in order to push spending up to build said war machine. Do it that way though, don't buy ships; I usually don't even alter the percentage going into military spending.

Build up your military put research into planetary invasion and build transports. While your doing that try to research down that line so you can win planets easily (it really sucks having to use two transports to wipe 5 billion people from a planet). Oh, and when your don't this make sure you take any good planets (naturally) and resources your enemy has. I generally don't finish them off though (but that's up to you).

The war phase is really important. It boosts your economy (through conquered planets). Also, you'll some of your enemies techs which is nice too. At this point I fortify and then decide where to go for victory.

Sorry, one more thing, a good way to pick a race to attack is to go for the race that's taking all of the crappy planets in your territory. Just start scouting their territory and start sending transports and support ships to their colonies. If you think you can, just go after the good planets. I usually skip the crappy planets they've settled in my territory if because if I have decent influence (as I usually do) they will rebel and join me anyway. Otherwise go after a reasonably strong race. You don't have much to benefit from by attacking weaker races.

Hope this helps....