Boom Beach puts another skin on the overused freemium concept, and that is the skin of beach assaults. Besides the standard upgrade-able buildings, in Boom Beach you will also need to construct and maintain an assault force.
Your base is located in an archipelago. Discovering and conquering this archipelago is essentially what you are supposed to do. Your combat force will always consist of a gunboat and a number of landing crafts that can be filled with land troops. In combat, troops are again choosing their own targets, but you do have some control over what's happening with the help of the gunboat. The gunboat can initially fire on targets of your choosing, but every round costs more and more energy (no surprise here). Energy is gained by destroying enemy structures, so even though your dumb troops fire at supply boxes while taking fire, even destroyed props do give an advantage as they can unlock another shot from your gunboat. I should also mention, that upgrading the boat will unlock more abilities including extra move orders.
Sadly, your troops will die sometimes, but they can be replenished at base with troops of the same type. If you managed to unlock new troop types, but you are lacking in boats, you can always discard the old ones. As impractical as that sounds, you will sometimes need this compromise, as landing boats are unlocked only with HQ upgrades, which, as you might suspect, take ages to complete.
The situation at home
Base defense is handled by buildings exclusively. With all the training and stress, your troops cannot handle defending their base. It is considered that they suffer from Selective PTSD, where they get queasy when it comes to defend their own beach, be it infantry or tank. This is also the case with your gunboat for some reason. However, beaches can be defended will all manner of gun emplacements and land mines. So essentially you will have a separate attack and defense power.
That might be interesting if multiplayer would entail people storming each-others bases. However, you can only join others in attacking a NPC entity called The Blackguard, which draws its aesthetics from the SS forces of National Socialist Germany and their psychological profile from Hollywood movies. The conquerable bases also belong to the same entity thematically.
Growing the economy
Base expansion on the other hand is fairly neat. You will initially have a decent amount of cleared land to build upon, but otherwise the island is covered in a thick forest. You can dig up any tree by paying gold and getting wood, which is nice for a change. Gold, the primary resource is generated by local residences (tents) that you can build, but also from the villages you've ahem... liberated from the Blackguard. The remainder of resources (stone and iron) will derive from their appropriate extraction buildings and are unlocked at higher levels, along with the buildings that require these resources.
Boom Beach has a very clean look. There's only one type of 'resource' that can be bought with real money and that feature is not shoved in your face. The interface is very clean with few buttons and clear meanings. The graphics blend 3D and isometric 2D with taste, while both individual islands and the archipelago are pleasant to look at.
What can I say, Boom Beach is a nice time waster. It's clear that Supercell has learned how to make the least annoying freemium game and that is a great achievement in itself.