Once you've written your first script, you've already at least seen AviSource. Might as well read up on it now, as well as its kinsman DirectShowSource. All the filters in the world don't help until you can get your videos loaded!
As with everything it depends on your source, but you'll almost always need to Crop your video to eliminate black bars or areas too noisy to "salvage." In a similar vein, BicubicResize (or BilinearResize if downsampling to reduce filesize) are extremely common tasks, though they usually appear toward the end of a script unless you're going for supersampling.
Those of you coming from the graphics world will recognize the next batch: Levels handles brightness/contrast/gamma, while RgbAdjust and Tweak let you do actual color correction in the RGB and YUV colorspaces, respectively.
At some point, probably sooner rather than later, you're going to run into interlaced video. I won't cover the general topic in detail here, but some websites that do include Lurker's Guide and 100fps.com. The hardware & software that inputs and manipulates field-based video on the PC is an imperfect science, so it won't be long before you'll want to be familiar with the likes of SeparateFields and Weave. Now all your old friends like Trim can manipulate fields directly. DoubleWeave and SelectEvery are even more powerful tools for recombining fields into frames as you see fit.
Once you've got an eye for the perils of interlacing, it's obvious that no amount of playing with fields will eliminate the artifacts that come from combining them (as we must on a progressive display) into frames. Time to learn about deinterlacers: a Bob filter is included, but as the 100fps site linked earlier indicated that's just the tip of the iceberg. To really do it right it's time to turn to Avisynth plugins -- TomsMoComp was a favorite but some recent filters are even more advanced.
But wait, the video industry has conspired to make interlaced video even more complicated: to broadcast (or otherwise distribute for NTSC audiences, e.g. on DVD) a 24fps film, it is telecined to make it 30fps. The Pulldown filter has a good description and can fix basic problems of this sort, but in the real world pulldown flags can be wrongly encoded, edits can be made improperly, sources can be a variety of framerates -- in all, you'll find it very nice to have an intelligent plugin like Donald Graft's Decomb on your side when it comes time to try reconstructing a progressive video.
Fine, so you've put the pieces of the interlaced puzzle together correctly; what now? Well, if your video is from an analog source it could probably use some noise removal. The standard filters are SpatialSoften and TemporalSoften; take some time to play with them. Many smoothing plugins are also popular, offering better control in return for a bit more complexity: try for example SmootherHiQ, a nice choice for starters.
The last stop on our tour of common filters is for advanced stuff: Layer, Mask, ColorKeyMask, GeneralConvolution, Overlay and Animate will have immediate applications for video gurus, while newbies are advised to read some of the many example scripts showing what can be done to create compositing effects, remove noise, and much more.