the search engine won't index the text "Appliance". This might be useful for the navigation, irrelevant or sensitive content. There are other ways to use these tags, and the idea is similar to the section targeting from AdSense.
Seeing an article from Wikipedia (now edited) that stated that Google uses these comment tags for the web search engine, Search Engine Roundtable asks if the information is accurate. Most likely Google doesn't use the flags, but they would be useful to improve the quality of the search results. Google should analyze if the flags are used for dubious reasons (prevent indexing most of the page content, hide spam).
I've seen many search results that are in the top positions because of the keywords from the navigation links.
Another idea would be to stop taking into account the keywords from the navigation links. Also Google shouldn't index the most part of a blog's homepage, archive pages and essentially any homepage that changes very frequently, as most of the times the content is available in separate pages (permalinks), so it's redundant. You also have to use the cache to see the page, as it shows different content now.
Google could also separate the content of a page in different clusters so if a page talks about more than one subject, each part will be treated like a different page. This way, the keywords won't be mixed and the different topics will be independent.
The first result for [windows live mail] is ideas.live.com page, and not the product's page, and that's because "windows live" appears a lot on that page.
A combination of flags and automatic content separating will be useful to improve Google Search.