The longer you play, the deeper it gets...
Positives: King's Bounty is very familiar - it plays pretty much like Hero's of Might and Magic without the need to build castles. Your focus is on your hero as he lends offensive and defensive bonuses to the armies and is able to cast spells from his undisclosed secure location.
As you progress, the game rewards you with some unexpected depth. You can marry and have children. You get a demonic cube accidentally welded into your hand which later allows you to summon allies. As the plot-line advances, you get to see some very, very large bosses. Yes, you get rewarded for progressing.
Combat has some fun elements. Berserk units can get a little too blood-thirsty and can attack your own units (if in range). You can discover entertaining combos like casting a trap spell and then using an Arch-magi to move an enemy unit into the trap to devastating effect.
The world is hidden and your first play through will give you the thrill of exploration. What are on the maps (rewards, especially) change with each of the 3 classes, so a re-play with a different class will indeed have some differences in the world as well.
Negatives: It is very easy get trapped in this game. You can run out of a particular type of unit(s), over-extend your cash flow, choose poor or ineffective skills to develop, and just basically get yourself in situations where you feel the need to start over. And I have started over 3 times now... ggrrrr.
King's Bounty: The Legend is fairly grindy. You will find yourself moving between zones looking for enemy armies that you can easily defeat and you have to do this both on land and boat. The game quite simply does not reward battles that evenly matched, which is a shame since these tend to be much more interesting than constantly stomping "weak" armies. Between spotty troop replacements, the cost in both time and money in rebuilding damaged armies, there is no incentive or reward for challenging enemies as strong or a bit stronger than your own forces.
All in all, King's Bounty: The Legend is a superior game. Just abuse the quick-save feature (F-5) and have fun :)