The "grindy" Science was addressed already, but I have a feeling that this is highly uneven throughout the career and turns into the opposite in the mid-game. My impression with the Career mode was that early in the game, I had to launch multiple boring sub-orbital flights to nearly all biomes of Kerbin to farm enough science to get into the orbit (BACC Solid Booster and TT-38K Radial Decoupler are a must). Back then, I was in constant shortage of science. I had to plan my advancement through the tech three very carefully to avoid buying secondary parts instead of the really important ones. Then I researched the Mystery Goo Containment Unit and the SC-9001, and that is when the career became a piece of cake. Armed with these two devices, I was able to bring massive amounts of tech from each mission. I crated an unmanned vessel with level 4 parts which is capable of going to Mun and Minmus and return. After several launches to different biomes at Mun (which were also necessary to test landing and return maneuvers, fuel required for them etc.) I am largely at tech level 6, even before I brought a single Kerbal to the Mun. Now, in my opinion, to fully unlock the tech tree, the player should have explored all or most of the solar system. Generally, in a sandbox game, the career mode is to prevent that the player has instant access to all of the "toys", so that he has some additional goals to achieve. But I fear that after my Kerbals have visited Mun and Minmus, the tech tree will be finished. Tl;dr: Science is hard at the beginning, but becomes too easy once you reach orbit. To improve this, it is probably necessary to fine-tune the costs of the techs and the amounts of science a mission yields. I, personally, would embrace getting more science from the mission itself at the cost of science from each biome. This would encourage the player to visit more celestial bodies, while detailed research of the biomes would be left for the case they run out of science at some point. I would also like to have Stayputnik or another unmanned module available right from the start. Rockets tend to crash more often at the beginning of the career. I don't like losing personnel, and when Reputation will be introduced, I don't want to get a penalty early in the game because I was unable to test my early rockets before putting a kerbonaut into them. What might also improve the situation is making spaceplane parts, thermometers and barometers to be available much earlier. At the moment, a spaceplane is, in terms of science, a pure vanity project, because by the time you research the required parts, Kerbin will be mostly explored by necessity. Why not letting the player to do so with an airplane? This would give them the possibility to get data from multiple biomes in one flight, eliminating the need for multiple sub-orbital flights before going to space.
