I don't think anyone should worry about the quality of the story and voice acting, as R* have proven themselves in those areas time and time again.
I'm mostly worried about the quality of the PC experience.
Yeah, Michael Hollick was named Best Male Voice Actor at the 2009 Video Game Awards, I think that says a lot right there. When you listen to him talk you'd never think he could do such a superb Serbian accent unless you already knew he was the voice for Niko Bellic. Michael added intense emotion to the part. It really felt like you were immersed in the Bellic family in missions like the one where he rescued his cousin Roman.
I'm not even worried about the quality of the PC experience because Rockstar is still surprising us with continued support for GTA IV PC. From what I've seen so far they've delivered on game fixes and word is the new GTA IV patch is even better, having even more fixes and optimizations that make the game run noticeably better. I'm looking forward to trying it soon.
They also showed us in GTA IV they can excel at making very nice cityscapes with realistc graphics and effects, like putting in nuances such as Euphoria physics enhancement. They are one of few developers that have used Euphoria and now they have lots of money from GTA IV sales to continue making nice games with a large development team having specialists in each field.