The overall game felt like it was set at normal or maybe a step above normal.
The beginning gave you a bit of slack but then became increasingly difficult. (IE: well balanced)
I felt that this game was very enjoyable, despite the learning curve. The game tested my wits at every level (except maybe the first few.) I eventually was forced to set up defensive positions and held out until the enemy mortors crushed my ranks. (Wave 9) I think that this game is very good as it is currently.
-tutorial as it took me a couple tries before I could make it past the first few levels.
-easier level for players who don't really want to be stressed
-the ability to select multiple units
-upgrade system (IE I kill 10 green Martians with my marine, he gets a slightly better gun)
-mines regenerate (after I mined those that I could, I felt the miners were useless except for suicide bombers 5dmg :D)
Some hints for newer players:
-Rush the aliens with your 2 marines in the beginning and warp a miner to steal their rock. Be prepared to heal your units and warp in more marines near the frontier. I found that healing was a must.
-Turrets have more range then their soldier counterparts but appear to have less accuracy. (+ they cost more ):)
-Missiles have splash, but they can hurt allies.
-(?)Each unit type has a weakness. (the missilers took 6 damage from my missile ret while only 3 from my sniper)