In the on access scanner setup you can add file extensions to “Scan files on open” and “Scan created/modified files”. But on the latter one you can select a “Default extension set”, why not on the first (“Scan files on open”). Isn’t it just as important to scan files on open? Or do I slow down my system when I add all extensions here as well (I copied all from the “Default extension list” to “Scan files on open” 
I think it is only logical to assume that the more files you scan on open the greater will be the resources of your system you will consume.
Also, I think it is logical to assume that the most likely source of problems encountered in the moment to moment running of your system will be from executable code (that is subjected to standard shield scan) and there are few problems to be encountered from the simple reading of data on your system (especially if that data has been scanned on create/modify and subjected to regular on demand scanning).
I cannot speak for the avast! team but I assume that is what leads them to the defaults of the avast! scanning methodology.
I choose to have my internet file download program scan all files (including archives ) that I download using ashquick.exe. I also run my scheduled on demand scans of my system using ashquick.exe which automatically scans archive files. So I feel comfortable avoiding the need to scan archive files on access. In the unlikely circumstance that an archive file has been contaminated since the last scan then I work on the principle that problems are most likely to occur only if I extract and execute code from the archive that will be subjected to the scan of the standard shield.
Igor stated here: http://forum.avast.com/index.php?topic=18295.msg156727#msg156727
This thread prompted me to look at the default list in the standard provider.
I notice that the default list of excluded files includes:
C:\WINDOWS\TEMP_AVAST_\UNP*
I assume that this is derived from the environmental variables of the system but seems to ignore that avast! now allows for that placement to be overriden by a registry setting. Shouldn’t this be adjusted by avast! if the registry override is in use or are we to be left to do it ourselves?
Do you mean something like
%windir% or %temp% ?
Well, the exclusion list compares text strings, even C:\Program files\ is different that C:\Progra~1
I would like if this (the adjust) could be automatic… :-\