Yeah you can wander around and go where you like, as long as you can survive. There's folk everywhere with sub-missions, and most of those are exciting to do (none of this go-here-talk-to-this-guy-and-bring-back-this-fucking-kettle pish from Morrowind). Some of the enemies are pretty tough (soldiers, bandits, terrifying monsters) and even the least dangerous areas can still be hazardous. You have to know when to run, when to sneak and when to go blasting through with shotguns and grenades. On many of the main missions you spend a lot of time tiptoeing around warehouses, labs, military bases listening for the scuff of boots around a corner, or zooming in with binoculars to check if that's a sniper's head poking up in the distance. This was the funnest part of SOF2 and Stalker does it better and without the bugs.
There's no fast travel like in Morrowind and Oblivion, so you do have to run everywhere, and the Zone is pretty big. Fortunately your guy runs very fast, though you need to take food to keep your energy up.