First off, a review of a core rulebook is pretty similar to a review of a game system, so this is basically that. The short version? I really, really enjoy Starfinder, and I think the Core Rulebook does an excellent job of explaining and showcasing the system.
Alright, the long version:
Character Creation: In comparison to Pathfinder, building characters is less fiddly while maintaining a large array of meaningful customization options. One new addition is Themes (analogous to Pathfinder's traits) which have a small amount of mechanical impact but do a very good job of hooking your character into the world around them.
Races: Starfinder ships with six new alien races as well as all the core Pathfinder races. Most of them largely fall into the category of Star Trek aliens (ya know, humans with masks on), but they have some fairly interesting racial abilities to differentiate them.
Classes: Starfinder has seven classes. Some are familiar (soldiers are basically space-fighters), some are deceptively different (technomancers look like wizards, but they're really not), and some are completely new (solarians are... solarians). Overall, the power curve is pretty tight and each class is interesting, highly customizable, and worth playing.
Feats, Skills, and Spells: Feats are, overall, less important than in Pathfinder (as a corollary, feat taxes have been eliminated; the feat trees are quite short). Skills are arguably more important (especially in starship combat), and the skill list has been condensed in a way that makes skills more accessible. The biggest change to the magic system is probably the addition of undercasting (i.e., some spells can be cast at multiple spell levels), which is absolutely essential, considering that both spellcasting classes in Starfinder are spontaneous.
Tactical Rules: It feels like Pathfinder. There are a host of small to medium changes (iterative attacks have been replaced with scaling weapon damage, for example), but if you like Pathfinder combat, you'll like Starfinder combat (unless, of course, you hate change - Starfinder is a new system, not a setting on top of an old system).
Other Things: Starship combat! I think it's pretty neat and makes a good addition to the game. There's also an expansive chapter on setting, which includes deities, planets, groups, etc. - it's usful to both players & GMs. There's a chapter on game mastering, which is nice. Finally, the art is fantastic through and through.
