The biggest differences are lighting, props and the input/output system in basic terms. There are many other changes but they're easy to figure out.
As already pointed out, it can be very prop based (as in
prop_static,
prop_physics and
prop_dynamic included suffix
_multiplayer and
_override based props). You can see what each prop is 'allowed' to be by looking on info in the model viewer. The
_override suffix for
prop_dynamic and
prop_physics can allow additional values for example, stopping a breakable crate from being broken on hit, etc. The
_multiplayer suffix forces props to go server-side and changes the collision settings (goes non-solid and pushes other props and players away).
In terms of lighting you have the difference in that lighting can be HDR, and also set to default as Quadratic lighting. Quadratic lighting just has a lower brightness in its falloff as seen here:
https://developer.valvesoftware.com/wiki/Constant-Linear-Quadratic_Falloff.
However, this can be altered in hammer by changing
Quadratic to "0" and
Linear to "1" (on the light entity). It's also nicer to use
light_spot in Source more than
light because of the shadow quality being much sexier in Source. That said, the effect of using a very dim
light and bright
light_spot produces nice results.
The input/output system is all done through the entity itself removing multisources, etc. The output and input has its own tab along with spawnvalues and flags.
If anyone needs any additional help, PM me or add me to Steam. There are a ton of changes.