I mostly played SC and AoM. I spent the last 5 or so years mastering LAN AoM (mostly Loki).
I'll describe what it's like to play an online match of DoWII.
You have a base. A pop cap of 100. A boss/captain/Warlock/Hive Tyrant/or other officer of the chosen army and one unit of light infantry.
You have some requisition to spend (money/teefs), energy starts at zero.
The req keeps ticking up (pretty fast) and the energy ticks up slowly.
There are three base objectives and a bunch of req. and energy objectives.
You have to capture the resource objectives, which make your stuff tick up faster and bases to make your opponents victory points tick down (500 to 0).
At certain points you'll upgrade your base to open up more units and upgrades.
The only thing to build is energy generators. (except some Tyrannid bosses can build tunnels and some bosses can build gun emplacements.)
The key is to hold positions and keep units alive (they can be replenished at the base).
Other than marines, the infantry is divided in to ranged and close combat, there's some type of heavy infantry, and then specialized units and vehicles (Dreadnaughts or walkers are pretty important but troop carriers and tanks can sometimes be more important).
The space marines are more heavily armored and can take on many of the other armies units but lack in close combat attack power (lower numbers cause this mostl). The light infantry are scouts, they are useless in combat (for the most part) but there are also heavy weapons teams and assault marines which where the heavy armor and are pretty expensive.
All the rest have pretty cheap infantry. Tyrannids ('nid) are the cheapest but the (living-)vehicles are pretty expensive but really tough. The Eldar have light vehicles but they tend to have high attack and are cheaper than the 'nids. Orks have cheap and weak vehicles but the multiple vehicles make for multiple targets so you can kick ass if you use'em right.
The 'nid walker (a Carnifex) is really expensive and is only available in the third/last base level (age). Once you have one fielded though they can kick a lot of ass. The orks have Nob squads (which are a whole squad of sgts.) and a tank. The S.Marines have a kick ass tank. The eldar have an Avatar (god) which is unattainably expensive.
Each unit can be upgraded with special weapons or spells, and sgts.