Well, I suppose the first thing would be, there are a few functions in the editor that, while available to use while editing in TF2, do not work properly in game.
An example of this would be the func_rotating. In CSS you could make something like a spinning disk with it, however in TF2, if you use it the brush does not move but if a player touches it, they will move.
There are various bypasses for getting around such problems.
In addition to some old entities not working, there are also some new ones, all of the TF2 props and materials are of course available too you in the TF2 SDK, (Hint: for getting to tf2 materials easily in the browser, type "tf" into the bottom field in the browser window) generally these work the same as in CSS and such, however you also want to be sure to minimize the usage of CSS/HL2 props an materials in TF2. On the same note, there are new game entities that are included, such as intelegence for CTF maps, Dispenser and Sentry entities that you can include as part of the map and set various settings of, there are also entities for controlling things like capture points and PL bombs.
One of the biggest differences between CSS and TF2 is the amount of team specific mapping is required, for example, triggers that allow BLU spawn doors to only open for BLU players and other triggers that prevent RED players from entering BLU spawns when they are already open. That sort of thing.
If you need more specifics I would be happy to help, and you should also be able to find many resources here, at the Valve Dev wiki, and on TF2 mapping communities.
Hope your potential "transition" to TF2 mapping is a smooth and fun one! :)
