One of the best ways to get a handle on what is going on in Master of Orion is to try to think of the game in terms of different stages of play. There are many ways to slice and dice this, but they are all worth thinking about in order to improve one's own play. The simplest and most straightforward way to go about it would be to base it off something as cut and dried as the research tree.
In doing this, you could separate out the game into 5 level blocks like the game already does. Each increase in tech level increases the cost to research that technology, not linearly but exponentially.
Researching a level 1 tech might be 300 research points, researching a lvl 50 tech is not 50 x 300 (which would be 6,000) but it is more like 200,000. The reason this works is because planet productivity increases greatly faster than a linear rate and so does colonial expansion. You might only be able to generate 20 - 100 research points a turn early in the game, but 60,000 research points a turn is possible.
Generally speaking, for most races, most people will spend on research fields approximately evenly for most of the game. This tends to lead to a climbing of the tech tree at a relatively even rate. It is feasible, then, to consider yourself in an "era" of a given tech level. Ex. A player might be in the tech level 1 - 6 era.
The player could then plan his actions primarily to combat other stuff in the same era, because thats probably what the opponents are using too if you are proceeding along at the same approximate rate as your opponents.
The "Up to Tech level 5 Era" might be characterized as having a lot of weapons that commonly miss, paying huge portions of production to alleviate pollution, very weak weaponry, only the ability to colonize normal terrain planets, and not being able to venture far away from one's planets with their ships.
The 6 - 10 era might be characterized as significantly less percentages of production spent on pollution spending, needing to develop cutting edge weapons to do any damage at all, only the easiest hostile environments being colonized, and the massive threat of the first death weapon when nobody is even close to having defenses to nullify it.
The most significant techs in the 11 - 15 range are Class V Planetary Shields and Scatter Pack V Missiles. Both of these make it very very hard to assault planets with substantial numbers of missile bases. They almost make planets invincible in this stage of the game and they really bring everyone into "defensive mode" and they signify the time to begin a tech war. The only way to assault planets beyond this point is to have huge leads in tech. The Bio Toxin Antidote is just outside of this tech range and is commonly researched earlier than its stage would signify because it plugs the massive hole in the inability of planetary bases to be assaulted once these two techs have been implemented - that of the death weapon, the only thing that will completely bypass planetary shields. The next higher death weapon, the Doom Virus, isn't available till the 26-30 range, and the Bio Toxin Antidote can almost guarantee you live to make it to that range, so it pays to get it as soon as it is available.
As a matter of fact, the Class V Planetary Shield is so game defining that the entire game almost hinges around you getting it. The entire game is marketedly different when you complete its research. Sadly, it can be randomly unavailable, and not having it randomly be available is probably one of the most direct causes of game losses on the higher difficulty levels.
Whatever method you use to define the progression of the game, you will find that to be absolutely key in every single one of them. Note also that there are whole races that are designed around different stages of the game. The special ability of the Silicoids, for instance, makes them very strong early and makes them lose relative power against the other races very quickly. They are probably 50% better early game and 50% worse late game.
The Psilons, on the other hand, get huge bonuses to tech research and are generally incentivized to get huge research leads as a way to dominate the other players. This makes them a "late game" race, except their late game occurs during your "middle game" due to their speed of researching. The Alkaris and Mrrshans are also much harder to deal with early than they are late, as are the Bulrathis.