Save the Earth tried hard to improve from its predecessor but it sucks. Yet it is a game that worth you a while.
Graphically, the monsters designs and the buildings have been more detailed but it hardly helps. Meaning, you won't be picking up the same building to knock down your foe all the time. The monsters' appearance have been slightly modified to make them feel like different from their looks in Destroy All Monsters. However, it goes with the expense of realism. As for myself, I think the monsters are like cartoon characters. The graphics in this game pretty much cannot hold up with the first game's.
The sound effects are,I would say, poor. I have no opinion on the monster roars but I do get disappointed with the degradation of the others. For example, the new air strike monster which assist you as long as you have that power-up does not make any sound. In the old game, Mothra gives a loud and clear sound when attacking. And the 'KO' announcement is awful to hear.
I have played through Action Mode for almost all monsters. I was dead unhappy to find mini-games between segments of fighting. The producers must have wanted to add some more value to the gameplay but I think they are time-consuming and of no fun. Some are quite difficult and may let you down if you fail to accomplish the mission. The excitement of fighting is completely washed away owing to them although It slowly grows again when returning to the fights.
What I really hate about this game is the point system. You fight to beat your previous high points by collecting G-cells, finishing off your opponent on time, destroying human defense and playing on higher difficulties. Please note that you gain only the 'point difference' between your new high score and the previous. So it becomes harder to raise points later on even though you get skills. (That's why you should buy cities and monsters first because they take thousands of points while gallery items are cheap.) That is very troublesome. You may find the monster you like most and control the best no longer effective in making points since you already have great high score and you are forced to use monsters you normally won't more than once against your will. Unlocking a arena pack or monster pack does not mean you can use the monsters in that pack. You must spend on each of the monsters in that pack that appear as question mark before you buy the pack again to use them. In short? damn it!
Talking about scores, looking for G-cells and destroying human army diverge your attention to kill your opponent and take you out of the action.
The only positive addition would be beam fighting. You can be very impressed winning a beam fight especially if you use Godzilla against Mechagodzilla. I suppose the damage of beam fight can be greater though.
Save the Earth is broken by many poor design decisions. It added a great many unnecessary changes and tremendous foolish elements to the gameplay. I would be happier to stick with Destroy All Monsters.