The first Just Cause had an abundance of flaws, like a terrible story, awful voice acting, wonky controls, extremley repeditive missions, tons of potential game killing glitches, and while it was pretty cool that you use the grappling hook to real yourself into vehicals, it was very disappointing that you couldn't use it for anything else.
However, it also had some moments of greatness, like a huge and heavily detailed map, a lot of cool weapons and vehicals, an infinitely respawning parachute, and it had a great level of immersion at times. Just Cause was more on the lines of a playground for players to just dick around in mostly.
Just Cause 2 seems to be a MASSIVE improvment over the first. The story and voice acting don't seem to have improved all that much, but after watching the first dev. walkthrough months ago, I was officially intrigued. I just hope to God they polish the shit of the game, that was one of the main problems with JC1. If they do that, JC2 could easily be the best sandbox game since Saints Row 2.
