Having played many different space 4X games, I've seen a lot of different tech tree/research implementations. One of the things that bothers me about a lot of them is tendency to have many 'incremental' research breakthroughs and very few 'revolutionary' breakthroughs. I'll explain. Incremental research results in an improvement on an existing technology. Larger fuel tanks, higher damage lasers, more efficient power generators, etc. These are, for the most part, an improvement of the numbers behind the tech. While these types of research are good to have (because they often result in a competitive edge), they aren't all that exciting or compelling. In Pax Imperia: Eminent Domain, for example, most of the research yields incremental changes, and it wasn't long before my interest in research waned. I found myself simply accepting whatever the game suggested as my next research, because I didn't feel that careful planning and selection of research paths would yield a significant advantage. Perhaps there were some revolutionary techs here or there in the research tree, but I didn't feel compelled to find them. Revolutionary breakthroughs, on the other hand, result in paradigm shifts (whether large or small) in how the game is played. With a revolutionary tech, new doors open, new gameplay strategies can emerge, and techs that were once strategically powerful start to look obsolete. Here are a few examples: 1) A new type of FTL drive. While all of the other empires are zipping around the galaxy using warp drives, one empire suddenly discovers the ability to create stable worm holes, resulting in instantaneous travel between two points. This is a huge game-changer. 2) Stealth technology. When an empire starts using cloaked ships, a wealth of new strategic choices opens up, forcing other empires to either research their own forms of stealth or re-tool their scanners in order to penetrate cloaking technology. 3) Super-fast tracking point defense turrets. Where combat used to be dominated by those who had the highest number of agile fighters, these new turrets will quickly render most fighter technology dangerously obsolete. The empire that focused its fleet around carriers will find that their combat abilities are greatly reduced. 4) Protein synthesis. We all know that agriculture and farming are the backbone of any powerful empire--citizens must be fed in order to stay healthy and happy. However, with the introduction of protein synthesis, food can now be created from common materials such as sea water or bedrock. With this technology, barren planets suddenly become much more viable for colonization! These revolutionary research techs are game-changers. They cause fundamental shifts in the way the game is played, and can sometimes tip the balance of power in the galaxy. It is these types of research that are exciting and compelling to the player. They keep the player interested in carefully planning out research paths instead of simply putting research on auto-pilot. In my opinion, a good, compelling tech tree will have both incremental and revolutionary techs, because both are necessary and helpful to a growing empire. However, a balance must be struck so that there are enough revolutionary techs in the tree to cause the player to make important, meaningful decisions about what to research.
SOTS is an interesting example of high end PD. Phasers and arguably PD missile as well rendered drones less effective, especially for races like the Liir that had a high chance of rolling them. The problem is that there needs to be a balance between PD and fighters. Otherwise you end up with the cruiser on cruiser fights of Gratuitous Space Battles 1.
