Governments:
Difference between governments has been emphasized. In conjunction with rebalancing buildings, this will force a player to think more carefully about strategic trade-offs. Religious buildings, and happiness generating buildings, may now prove critical, especially when conquering new provinces, or when an absolute monarchy attempts to modernize.
Absolute Monarchies:
An absolute monarchy is a state ruled by an oligarchy of individuals committed to the status quo, and fearing change. Change will only reduce their power and wealth. An absolute monarchy resists change and the destabilizing ideals of the enlightenment, and while considered backwards by the proponents of change, sometimes the old ways are best. Austria was an absolute monarchy.
An absolute monarchy will find it very difficult to significantly industrialize, due to the penalties it receives on lower class happiness. An absolute monarchy will need to rely on taxing the upper class, rather than its impoverished and unhappy lower class; this will have the general effect of stifling economic growth. An absolute monarchy will also need to carefully direct research -- too much focus in 'enlightenment' technologies, too many schools, or too much investment in modernization will cause the lower classes to take up arms. An absolute monarchy receives a penalty to its overall research speed, due to the lack of free-flowing ideas. However, an absolute monarchy receives a bonus to the recruitment cost of its units.
Constitutional Monarchies:
A constitutional monarchy is a state that must be ruled by a leader who can delicately balance the desires of the lower classes and the peers that represent the interest of the elites. A constitutional monarchy allows progress -- but not too much, nor too fast. Britain was a constitutional monarchy.
A constitutional monarchy is a balance between a republic and an absolute monarchy -- but it makes no one truly happy. The peers do not have the power they want, while the commoners do not have the freedom. A constitutional monarchy receives a small bonus to research, and another small bonus to the cost of recruitment. Constitutional monarchies will be able to achieve a reasonable amount of industrialization, or enlightenment technology, but must still monitor and control these areas. A constitutional monarchy walks a fine line between two extremes, but must careful balance and monitor the rate of progress.
Republics:
A republic is a state ruled by the tyranny of the many. Innovation and progress run deep in its marrow. Without rapid innovation and industrialization, however, the rights of man will wither and die on the vine. France after 1789 was a republic.
A republic is the most innovative type of state government, and it receives a bonus to research. As a republic can rely on a higher level of industrialization, and a happier lower-class, it need not overly tax its elite -- allowing for greater economic growth. A republic, with its free-thinking populace and quaint "one man, one vote" ideas, must put more effort into recruiting and training its soldiers and sailors, and therefore receives a penalty to the recruitment cost of its units. A republic that is not able to industrialize quickly enough will find itself unable to support its foreign wars.
Research:
The rate of research has generally been adjusted downwards, by reducing the spawn rate of gentlemen, reducing the research points generated by different school building types, and adjusting the government type research bonuses so they are normalized at -10% (i.e. technologies require +10% more research ponts).
Additionally, the 2nd and higher level of research traits acquired by gentlement will generate a negative effect in an opposing research type. Higher levels of specialization in Industry will reduce ability to research Enlightenment. Higher levels of specialization in Enlightenment will reduce the ability to research Military. Higher levels of specialization in Military will reduce the ability to research Industry. You will now to take this factor into account when assigning research to different school, and to different gentleman. If a school must research a particular technology area, you may want to move your specialists elsewhere in the meantime. Likewise, it will be beneficial to have schools specialize in research areas.
The destabilizing tendency of schools has been significantly increased over vanilla. Absolute monarchies, and to a lesser extent constitutional monarchies, will have particular difficulty with provinces that have these institutions.
Finally the cost of schools has been increased, discouraging the early game tendency for the player to just build a lot of schools, and max-out research early on. This will reduce how often the CAI build schools, but will have the same effect on the player.
All of these together will serve to significantly slow the cost of research. I prefer a mixture of subtle adjustments, especially those which add depth to the game play, over a simpler solution such as simply doubling the cost of researching a given technology.