And now for a CO biased review..
In NS you have the ability to build buildings. Resource Towers and Resource chambers(for

and their kin) gather resources, like the name suggests, but can only be built in very specific locations. Those resources can be used to make buildings, or, for aliens, get an upgrade, or change lifeform. For marines, they can be used to buy equipment like a shotgun, or upgrade stuff, like Armor level 1. Also, your upgrades are limited to what upgrade chamber type(there are 3, Defense, Movement, and Sensory) a gorge has dropped, if no chambers are up you will not be able to upgrade. You need 3 chambers of one type for the upgrades to be at their maximum effectiveness, any further beyond that will only provide backups and the chamber passive boost(ex Defense, it heals everything inside a radius of the chamber with 10 health per 2 seconds.)...
Also, you only have 2 weapons as an alien in the beginning. Marines only need to get an armory and then upgrade it to be able to BUY new weapons. Aliens need to build more hives, which as well as providing your "second hive abilities", will let more aliens respawn at one time, and will heal everything inside it's radius with 15% of it's maximum health(max of the object that is being healed, that is). You notice how hives are pretty important.
Oh, and only gorges(

) can make structures. So you have to rely on your team being smart and not n00b if you want to have fun(because losing as an alien is the most un-fun thing I have ever experienced).
And...Combat mode. Here, everything relies on you and your own skill and intelligence. If you hang around teammates with more skill you should gain levels pretty fast anyways, since experience(which is gained everytime an opponent is killed) is shared in the formula of, +10 to total sum for every teammate within range, divided by the amount of teammates and then doled out equally.
Alien teammates might not appreciate it if you ruin their hiding spot though, so don't run to where they are hiding right after attracting the marines attention. Chances are they will hate you.
Also, you get experience for damaging the enemy main structure- if youre an alien, go for the command chair, if youre marine, go for the hive. Oh, and there is no commander. And gorges cannot build anything. There are no resources either.
You need to level up to be able to upgrade, or get a new weapon. There is a bar at the bottom of your screen which shows how close you are to your next level, and what level you are. If there is a + anywhere in it, you can get upgrades or new weapons.
Oh, and if you die, you respawn with the choices you have made. So don't choose an upgrade you're not absolutely sure you want.
Some items cost more than 1 level. For example, going fade, you would need 3 spare points, one to go gorge, and 2 to go fade(you dont have to spend your available points as soon as you get them).