For those who just want to see and not read.
Made by Oxeye Game Studio (with a programmer who also works with Mojang on Minecraft), Harvest: Massive Encounter is different from other TD games I've played. There are, by default, 5 game modes:
Normal - In which endless waves of aliens come to attack your base and you must survive as long as you can and try to beat your own high score. (Also leaderboards.)
Wave - In which you must defeat 10 waves as quickly as possible. Perhaps also leaderboards and personal high score.
Rush - In which you must deal 50,000 damage as quickly as possible. Perhaps also leaderboards and personal high score.
Insane - Like Normal, but harder.
Creative (i.e. Sandbox) - With lots of different options, and which can be extended or modded using LUA.
So far I've only played Normal and Sandbox, but here's how the game is different from other TD games I've played before:
First of all, aliens don't travel along a pre-defined path; they attack from all sides and can destroy any building. This changes the strategy compared to typical TDs where the only strategy is only building placement (to block off paths or create bottlenecks) or when/what to upgrade next. In fact, the more I think about it, I suppose that Harvest is more similar to a simplified RTS. You don't get money just from killing aliens. You need to build harvesters to grab the minerals sitting around on the surface of the planet. Also, there's another resource besides money you have to worry about: Power. Every building you build requires not only money to buy, but power to build and in most cases power needs to be regularly replenished (e.g. your harvester will power down after mining a few times and needs a recharge before it can harvest anymore).
To make matters even more difficult, you can't just build a power plant to get an extra supply of power. The power plants generate electricity which then must be transported by hopping from node to node (called Energy Links) to reach its destination. This transportation process takes time, and requires careful planning of your Energy Links. For example, if you've got a defense tower on the outer edges of your base and it runs out of power, it will be useless until it gets some charge back in it. But that's not as simple as it seems, since a unit of power traveling down the nodes toward the defense tower can be grabbed by anything else that needs it along the way, essentially preventing the defense tower at the end of the line from ever getting any power.
Now that I've made it seem really hard, rest assured that there are ways to curb the difficulty. You can set up specific routes for power to travel and of course you can build power plants closer to the places that need it. You can also change game speed at any time from paused/stopped (during which time you can still look around and build things), slow motion, normal speed, double speed, and quadruple speed. Or, to put it more succinctly: 0x, 0.5x (?), 1x, 2x, and 4x. And don't forget to click the Attack Priorities button to customize which tower types should give which aliens the highest (or lowest) priority.
Due to the nature of Harvest's resource management, you can't just hunker down in one place and wait for the enemy to arrive--which is pretty much how all other TD games work--you constantly need to build more power plants and expand your base outward for more minerals. This while the enemy waves are becoming more and more difficult. And as you build toward the edges of the map, the map expands further in that direction.
It sounds very complex and yet the rules are fairly simple. There are basically only 5 buildings you can build: Solar Plant, Energy Link, Harvester, Defense Tower, Missile Turret. None of these can be upgraded except for the Missile Turret, which can be converted into a multi-rocket launcher called the Tempest Turret with EMP-like effects (visual only?) or a super-long range multi-rocket launcher called the Eagle Cruiser Turret that will sometimes fire at aliens before you can even see them on screen.
It's kind of hard to explain, and I think it sounds very difficult the way I'm explaining it, but it's pretty easy to grasp the basic concepts after a round or two of the game, and then the challenge of devising a good strategy comes next. I still don't feel like I have a good strategy, but there's always Sandbox mode to let me experiment.
And speaking of Sandbox mode, as I mentioned, the game is moddable with LUA so you can download mods from the community (here's a mod recommended by one of the game's programmers: Doublevil's Tower Defense Mode which makes it more like a traditional TD game) or you can even create your own mods to customize buildings or create new ones, etc.
All in all, Harvest seems to be deep, fun, and challenging. And though I haven't tried it, I imagine it would even run pretty well on less-powerful machines such as netbooks.
You can buy it for $10 from Oxeye Games or on Steam, and perhaps other digital distributors as well. I recommend it, but if you're still not convinced, by all means try out the demo before buying it.