If any ASP.NET novice developer (or any other programmer) needs to learn and explore SharePoint Designer, what prerequisite skills are required to learn and to be an expert in SharePoint Designer?
It very much depends on what you want to use SPD for. As it can be used for as different things as:
If you want to be an expert on all then you need to learn (in somewhat prioritised order):
Install the SharePoint
From the Central Administration create a site and play with the sites. Cover content, search, sites, communities, themes, master pages, page layouts etc.
The basic point here is, you will get a broader picture of What SharePoint and how it works. This will be a very good starting point since you're novice in ASP.NET/SharePoint
After couple of days, download SharePoint Designer 2010 and start practising most of things (as mentioned by Per) Workflows, Business Connectivity Services, Content types, Lists, List views, Ribbon buttons Pages, Advanced web parts, Page Layouts, Master Pages.