God of War 1 is about 6 years old now, so a lot of the elements/game mechanics/combat are exactly that, 6 years old and from a console of the past.
God of War 2 is a lot more polished. The graphics are upgraded some, the combat is very much improved, etc.
God of War 3's combat, however, blows your effing mind. It's quite possibly my favorite game ever when it comes to pure combat/gameplay. The graphics are some of the single best out of all video games, and the boss battles cannot be topped.
So, it's tough to judge the series going back and playing the first game and trying to compare it to new games, however, if you play the third one first (which I don't recommend simply because of story) your mind will be blown.
Beat the first game, then when you play the second you will see/feel the improvements right off the bat, then go into the third and have one hell of a time.