If you are moving to FDT from FlashDevelop and want FDT to look and behave more closely to FlashDevelop, here's what you need to do.

Open Window > Preferences.

Under FDT > Editor > Code Assist copy and paste this into Auto activation triggers for AS, and set the delay to 0ms.

abcdefghijklmnopqrstuvwxyz_. :

Under Editor: Colors and Editor > > Semantic Highlight you're going to set the syntax colors. This is a bit involved. What you should do on Windows is create a custom color in the palette for each of these.

Here are the RGB values for each of the items you need to change under Colors. You should turn off Bold/Italic for all these items if you want to match FlashDevelop's defaults.

Keywords and all Keyword 'types': 21, 24, 255
Base Types 'Void': 0, 128, 128
Core Types from MM Classes Root: 0, 128, 128
Constants: 0, 0, 153
Single/Multi Line Comment: 0, 128, 0
SingleMulti Line Todo: 0, 128, 0
JavaDoc: 128, 0, 0
Strings/Characters: 163, 21, 21

And here are the RGB values for each of the items you need to change under Semantic Highlight (Click on the AS3 tab).

Class: 0, 128, 128
Field: 0, 0, 155
Function: 0, 0, 0
Getter: 0, 0, 0
Regular expression: 255, 0, 255
Setter: 0, 0, 0
Static field: 0, 0, 0
Static function: 0, 0, 0
Static getter: 0, 0, 0
Static getter: 0, 0, 0
Top level function: 0, 128, 128
Top level namespace: 0, 128, 128
Top level variable: 0, 128, 128

Under Problems > AS3 Problems:
Unresolvable > Unresolvable variable reference in E4X and Unresolvable member reference in dynamic object should both be set to Disabled.

If you're on Mac you can skip this step. Windows users should definitely do this. Under General > Keys find Quick Fix (type it into the top input box), click on it (should be bound to CTRL+1). You can add CTRL+SHIFT+1 if you want to make it like FlashDevelop, but I would leave it alone. However, I highly recommend adding ALT+1 or ALT+2. It's a more natural position for your hand to be in.

Apply all your changes and return to the FDT IDE.

On the left you should see the Flash Explorer panel. Drag that entire block, not just the panel tab but the entire block, to the right edge of the application. This will mimic the Project tab's position in FlashDevelop. I would take the Outline and ANT tabs from the other block and add them to the same block as Flash Explorer. I closed the ANT tab since I don't use it often enough to warrant it being always visible.

That's basically it. The rest is all up to personal style.