I would suggest you take a look at this page on MozTrust. As the name indicates,  MozTrust is an tool which measures trust factors for a website.

MozTrust and PR are similar metrics. They are both attempts to determine a site's importance and credibility. The largest factor is your site's ability to earn credible external links from other credible sites.

Example 1- you create a natural viagra-like sex supplement using common ingredients from other similar pills. You set up an e-commerce site and sell your product. It will likely take you a long time (i.e. years) to build up trust unless you pour an enormous amount of resources (i.e. money) into the site.

Example 2 - you create a natural viagra-like sex supplement based on credible research from UC Berkeley or another credible institution. You perform authentic studies by doctors and the results are published in the New England Journal of Medicine and other credible medical journals. The doctors and researchers involved in the study all post numerous articles on your site, and respond to questions.

As a result of the above activities, the New York Times, CNN and other credible news sources cover the story and link to your site. Additionally the doctors involved with the study are asked to be interviewed on Oprah and other television shows. All the media hype turns into hundreds of links from highly credibly sites and a lot of social media buzz.

The second example can help a brand new site very quickly earn a lot of trust. Then the product begins selling, authentic testimonials are received, further research is performed, more doctors and patients begin working with the product, leading to even more credibility and trust.

I did a lot of research on this topic for you Storwell. I read articles from Bing, Duanne Forrester, Q&As on Bing Webmaster's area and checked a few other sources as well. There are several non-credible sources that discuss the topic so be careful if you go searching online for the answer. There is even an article on Sphinn which is titled "Bing Says Goodbye to META Description as a Ranking Factor". The article simply has no credibility and serves as an example of what leads to so much confusion in the SEO world.

Bing does not definitively state meta descriptions are not a ranking factor. It would be my best guess that Bing either does use meta descriptions as a ranking factor or it is an extremely low weighted factor.

Indirectly, Bing does weigh CTR as a ranking factor and a meta description does influence CTR so it can have an effect that way. In this sense, I would suggest writing a meta description tag for users. Write the most compelling and accurate description you can which will entice readers to click-through to your site. I would not make any attempt to modify a meta description to improve rankings. Focus CTR.

HTML sitemaps are generally designed for users, where XML sitemaps are designed for search engines. While either party could use either form of sitemap, each format is optimized for their intended audience.

If your site has proper navigation set up, a sitemap has almost no value at all. The modern day value of a sitemap submission to search engines is to alert search engines to web pages they would not otherwise locate during a crawl.

250 products seems like a lot to present on a page for an average user. Many product pages offer numerous links for each product for image viewing, examining the main product page, options such as colors, etc. A page with 250+ products seems unfriendly to most users and search engines, as I imagine the page may have 500+ links.

How long is the period for google to trust a completely new URL?

Trust is earned over time with links. Some sites will gain it very quickly, while others will never achieve high levels of trust.

The first question is, how exactly do you define "trust"? You could use PR to measure trust, but everything is relative. If you only consider a PR 10 site as trustworthy, it is 99.99% likely that your site will never be trustworthy. As of Aug 4th, 2011 there are only 14 PR 10 websites (pages) worldwide, yet there are tens of millions of websites.

Even if you establish a certain level as trustworthy, such as PR 7, the next issue is measuring PR. Google only releases PR toolbar updates 3-4 times each year, but the figures are updated daily internally.

If you decided PR 7 was your goal (as an example) then it is possible to achieve a PR 7 site quickly if you could pump enough resources (i.e. money) into the site. If you created a well-designed, quality site which offered a product, service or information that was credible enough to cause enough interest, then it can certainly be done.

do you think it is more important for a site to get say, 20 backlinks from anywhere. Or 5 backlinks from very trusted blogs, etc.?

I would prefer 1 quality link in content from a trusted blog or other quality source then 100 "backlinks from anywhere".

In my experience a 301 redirect is always the superior course of action. One reason is with a 301 redirect, you will ensure those who create links to your site will use the proper URL format. This way, your links go directly to the proper page without losing any link juice to a redirect.

Canonical tags are a great backup in case something goes wrong, but 301 redirects are always preferable.