Unlike Saints Row 2, development for the PC version of the The Third was not done by a 3rd party, and it shows. From a technical standpoint, it's a much smoother and less problematic game than Saints Row 2 was.
Missions feel a bit shorter, and there is no longer any Respect requirements for storyline missions. It can make the game itself feel shorter, as you no longer need to spam side missions (like Mayhem or Insurance fraud) to progress the storyline, They are, however, useful for gaining Respect which is still used for other things. The storyline missions are quite varied, and fun, and there are several that just makes you go "wtf?".
The storyline in The Third is definitely ligher than in SR2. While less "serious" than GTA4, the story in Saints Row 2 had some rather serious and downright depressing moments. Saints Row: The Third have no such things, or at least keeps it to an absolute minimum. One thing that has not changed is the Boss (or the protagonist, for anyone not familiar with the previous game). S/he is still an evil psychopath.
Character customization options are fairly similar to Saints Row 2, though it overall feels quicker and easier to use in Saints Row: The Third. Some options have been removed (for example walk animation and facial expression), some of the voice sets have been replaced with new ones, and there's new Zombie voice set for zombie fans.
Co-op is similar to how it was in Saints Row 2, except that its no longer an absolute pain to set up on the PC (we eventually had to get the Xbox version to get any co-op done). Setting up a co-op game in Saints Row: The Third is a pleasure in comparison, though it seems to be tied up completely with Steam. When, at one point, I lost my connection to Steam, my co-op partner got disconnected from my game, and a mission automatically failed as a result.
Overall, it was a better game than Saints Row 2 and, in my opinion, money well spent. It's not a perfect game but when, at the end of the game, I first thought was "This was fun!" I can not give it anything else than highest rating.
If you're a fan of Saints Row 2, get it now.
Played on: Intel/NVIDA/Win7 system. Co-Op over Internet.