The Valve Development Community
First and foremost, The VDC, is your #1 resource for Source SDK questions, and exploring new features and theory. Additionally, there are ALWAYS links at the bottom of each article, if you want to expand your knowledge further.
Materials
Textures are handled a little differently, but nothing you can't adjust to in 5 minutes fucking around. Soon as you want to mess with custom materials, head to VDC for a full overivew on materials, VTF, and vmt files, and get yourself VTFEdit by Nem, if you don't already have it.
Lighting
Texture lights exist in source, but i would recommend shying away unless you have a specialized use for them, e.g., lighting the underside edge of a door, but mostly you are going to want to get used to using pointlights, light_spots, and of course the light_environment entity is still there. (i'm still having a hell of a time with lighting, but CSGO lighting is so pretty, you don't have to work to hard to get very nice results)
Models
Most of your models in game are going to be prop_static. If it's something your want a player to be able to move or break, then use prop_phsyics(prop_phsyics_muliplayer for cs or dm games). If a static model is not showing up as a prop_static, then try it as a prop_dynamic, and it should then work. (There is a way to tell whether models are dynamic or not in the model browser, but i can't remember how atm)
Displacements
Displacements are awesome for many reasons, probably you know their primary reason to exist is to create beautiful, smooth terrain, quickly and cheaply for the engine to render, but they have other uses. A big one for me is that on huge maps, you can increase your brushwork budget by changing ordinary walls to displacement surfaces. Remember Displacements don't block VVIS, nor do they seal your level.
Overlay Tool
Info_Overlays are the newest way to apply decals, and they are awesome. VDC says they are more expensive to use than decals(called Infodecals in Source), but personally, i never, ever use infodecal anymore, mostly becasue they are buggy. Overlays can be moved and rescaled as much as you want and are much more stable than infodecals. Important infodecal note: if you alter or move a brush with an infodecal on it, it may "disappear". if this happens, simply double click on the little entity box thingie, select the Brush Faces keyvalue. Then, click on the "Pick" button, and then select the brush(or ctrl+select multiple brushes) that the overlay is to be applied to, and finally, hit "Apply". The overlay should now show up again.
VisGroups
VisGroups are MUCH improved in Source, and can easily help you organize bigger levels into more-manageable chunks. If you don't like corralling your map's brushes and entities into groups, there's always Auto-Visgrouping, which automatically takes every type of brush, tool brush and entity, and gives you toggles you can check/uncheck. Finally, a nice new addition to Source VisGroups are the quickhide/quickunhide toggles on the tool bar, that will make your mapping life much easier when you learn to use them.
Smoothing Groups
Besides using texture alignment to make brush faces flow together, you now have smoothing groups. If you have an area where brush edges don't seem to fit together, making them share a smoothing group will help them blend tremendously. (i don't uses these too often, since the stock smoothing is so good, but i'm sure i could use them more to good effect)
Cubemaps
Use cubemaps to control and tweak all the reflections in your map. These are another thing i never really learned to use too well in Source, but mostly, i just put one cubemap at eye level(80 units?) in each distinct area of the map. However, I've heard you can assign individual brush faces to each cubemap, directly controling what faces reflect, though I've never done this myself.
Compiling
Compiling in Source SDK should be a seamless transition for you, since all the options besides HDR exist. There are still "Normal" and "Expert" compiling of course, but 99% of the time i use Normal(fast VVIS, fast VRAD and HDR), unless i'm working on final lighting. An important compiling note: ALL NON-VISBLOCKING BRUSHES IN YOUR MAPS SHOULD BE FUNC_DETAIL! If not your compile will be bogged down by unecessary VVIS calculations, which will dramatically slow down your compile, and zero benefit to performance. Compiling is MUCH faster for Source SDK all-around if you follow this simple rule, even on the biggest maps!
Notes on Brushwork
As you may have heard, Hammer 4 sometimes has "issues", keeping complicated groups of verticies exactly where you left them, but take heart, the superior smoothing in Source makes it likely you won't notice very slightly off-grid points in the game, so my advice: don't obsess too much about grid points unless you are getting a visual error in the game. Important Brushwork Note: If any brush begins to "act strangely", e.g., when trying to clip a brush and it refuses to let you split it into 2 equal halves, scrap that brush and start over!
General Hammer Tips
If any tool or hotkey or anything in hammer starts acting "weird" or other than expected, immediatley SAVE your work and restart hammer! Also, even though hammer autosave works pretty good, it is by no means bulletproof(meaing it simply fails to work sometimes), so my strong advice is to regularly make backups of your cherished hard work!
That's all i can really think of for now, but don't forget: the most important reference tool for Source SDK users is The VDC!
