Interlaced versus field-based video
Currently (v2.5x and older versions), AviSynth has no interlaced flag which can be used for interlaced video. There is a field-based flag, but contrary to what you might expect, this flag is not related to interlaced video. In fact, all video (progressive or interlaced) is frame-based, unless you use AviSynth filters to change that. There are two filters who turn frame-based video into field-based video: SeparateFields and AssumeFieldBased. More information about this can be found here.
The Chroma Upsampling Error
The Chroma Upsampling Error is the result of your video is upsampled incorrectly (interlaced YV12 upsampled as progressive or vice versa). Visually, it means that you will often see gaps on the top and bottom of colored objects and "ghost" lines floating above or below the objects. The 4:2:0 Interlaced Chroma Problem is the problem that 4:2:0 Interlaced itself is flawed. The cause is that frames which show both moving parts and static parts are upsampled using interlaced upsampling. This result in chroma problems which are visible on bright-colored diagonal edges (in the static parts of the frame). More about these issues can be found here.
About the different RGB <-> YUV color conversions.
Wrong levels and colors upon playback
When playing back video content, several issues might go wrong. The levels could be wrong, resulting in washed out colors (black is displayed as dark gray and white is displayed as light gray). This is described in more detail here. The other issue is a slight distortion in color (which often looks like a small change in brightness) and this is described here.
There are two kinds of video when considering framerate. Constant framerate (cfr) video and variable framerate (vfr) video. For cfr video the frames have a constant duration, and for vfr video the frames have a non-constant duration. Many editing programs (including VirtualDub and AviSynth) assume that the video has cfr. One of the reasons is that avi doesn't support vfr. This won't change in the near future for various reasons. Although the avi container doesn't support vfr, there are several containers (mkv, mp4 and wmv for example) which do support vfr.
It's important to realize that in general video is intrinsically cfr (at least in the capping video or ripping dvds arena). There is one exception where converting to vfr can be useful, which is hybrid video (with different base framerates for various parts). The most common example of hybrid video consists of parts that are interlaced/progressive NTSC and other parts which are (telecined) FILM. Other examples of hybrid video include many of the modern anime TV Series, many of the Sci-Fi TV Series.
More info about creating vfr video and opening it in AviSynth can be found here.
Automatic interlace detection
Everything about automatic interlace detection.
Opening media into AviSynth
A lot of media formats (video, audio and images) can be imported into AviSynth by using one of AviSynth's internal filters, specific plugins or DirectShowSource in combination with the appropriate DirectShow filters. It is not always trivial to import your media into AviSynth, because there are often many ways to do so, and for each way you need to have some specific codecs installed. This document describes which formats can be imported in AviSynth and how they should be imported. Also a short summary is included about how to make graphs (graphs of appropriate DirectShow filters which can be used to play your media file) in Graphedit and how to open the graphs in AviSynth.
More about Format Conversions.
Resampling is the process of converting a signal from one sampling rate to another, while changing the information carried by the signal as little as possible. When applied to an image, this process is sometimes called image scaling. More about image scaling, various resampling kernels and the implementation in AviSynth can be found here.
High bit-depth Support with Avisynth
Currently AviSynth only supports 8-bit video. Nevertheless, there are several plugins and utilities which can provide support for high bit-depth processing, import and export for encoding.
Functions include color correction, color space conversion, debanding, denoising, dithering, resizing, and others. See High bit-depth Support with Avisynth