So I'll start off on the negative points.
I'm 10 mins into play and I've already got the trademark Fallout freeze up! You would of thought they'd have tried to improve on the freezing issue from FO3 but apparently not...
Instantly it's clear there's huge potential for replays in the story mode, but the skill tree is almost identical to FO3. It's clear that at the end of the game you will have nearly 100 in all skills and all the best equipment for all styles of play. Therefore just like with FO3, I'll be thinking, "yeah maybe I'd like to replay and follow a different story path, but I'll end up with exactly the same character as I had in my last play thru." That said there's a hardcore mode which may bring some variety...
As for improvements:
You can finally aim down the sights!
To conclude, if you've never played FO before than buy this one, it's the most ironed out, you'll love it. However, if you played FO3 it's the same car with a new paint job. Don't expect any major changes, but aiming down the sights of a weapon is a miracle!