We (try to) use keynotes as well, and ArchiCAD is a little behind on this. We're hoping that version 13, due out in the summer, may finally tackle the issue. In theory, ArchiCAD has all of the groundwork you could possibly need to do this and more. You can embed information in each part, link that information to a database, and even interactively scheudle much of that information. Unfortunately, not all of the dots have been connected. Cadimage Tools has the best solution.

Some other discussions about ArchiCAD and keynotes:
http://archicad-talk.graphisoft.com/viewtopic.php?p=140062&highlight=#140062

Regarding your other questions about ArchiCAD:
1. Dimensioning is awesome. stings are a single entity, always associative, and very reliable.
2. Site stuff is tough for most "Architecture" packages. Applying treatments to a non-flat base plane involves a whole new set of tools. ArchiCAD has decent meshes, but if your site isn't pretty flat, you'll need an add-on, like ArchiTerra by Cigraph (another purveyor of add-ons, like CadImage, but Italian instead of Kiwi). Then it's pretty good.
3. Though no full-featured word processor, I've never had a problem (tabs, formatting, and autotext fields work well).
4. The 3D documentation thing is new and wonderful. It's a prime example of Graphisoft listening to their user base!
5. I hear there's some revamping going on, but the material editor in ArchiCAD has always been pretty intuitive. Lightworks makes it a bit more complicated. It's not as good as 3DStudio or Parenesi, but then again those aren't BIM.
6. I believe the RoofMaker and TrussMaker utilities still come with ArchiCAD, and there are free downloadable "Goodies" available in your Help menu (used to be included on the DVD), including the Accessories, that have, among other things, a Wall Framer. These are a little tricky to master, but if you use them all the time, they're very functional.
7. With a lot of options for layouts and viewports, plus support for PDFs and about any image format, I've found it a good platform to do presentation boards. It's also pretty easy to create fly-through movies, do real-time 3D navigation, or create Quicktime VR Panoramas and Objects. Lightworks rendering is, in my opinion, pretty decent.

One other thing I've heard from Revit users is that it's a resource hog. Several firms have had to buy a lot of new computers just to run it. ArchiCAD (especially 12, which was tweaked for speed) will run acceptably on a Pentium 4 with 1GB of RAM. It flies on a Core2Duo.

As far as GDL, you really don't need it, at least not all of it. There are degrees of object making, and you can do a lot without typing a line of code. You can make vignettes of objects you need using walls, slabs, and even other objects, then save as an object, lamp, door, or window. With a tiny bit of GDL (like "ROTX 90"), you can make it do about anything.

However, if you play with the available parametric objects, you can probably make what you need without modification. For Nanawall (I love that stuff, but it's expensive), you can just use a bifold door, give it full-height windows, and you're there. Who cares that the object was intended to be a closet door? Architects often alter things beyond their intended use.

If you do want to get into GDL later, it's a really easy language. Most of it is based on BASIC, which I learned in 10th grade (it amazes me that they let people out of college without a programming class!). In my opinion, it's easier and more robust than LISP or ARX, or even VisualBASIC. It has commands for geometric objects, but you can cheat... create the object in ArchiCAD and have it generate code that you can copy and paste. The trick is to start small, and figure out what you really want before creating it. There are a lot of resources out there.

Good luck coming over to the Dark Side!