Ignacio, your question reminded me of an interesting White Board session from Randy Fishkin of SEOmoz in the Year 2009 in which, he talked about Crawling and Indexing.

Here is a summary of that whiteboard session and I hope it will answer your question(s).

The crawlability of the website is based on the following factors i.e.

• Links (How your site page are linked to each other or in other words, how your link tree has been sculpted?)
• Freshness/Updates of your site content and it's update frequency
• Feeds (Pinging the search engine about your updated feeds)
• Domain Importance (Linkage of your domain to other popular and trust-worth domains)
• Viability (Usability of your website and how your link structure has been sculpted)

The indexability of the website is based on the following factors i.e.

• Uniqueness/Value of your site content
• Domain Importance (Linkage to popular and trustworthy domains)
• Raw Link Juice (How many links are pointing to you internally & externally)
• External Links (Any votes you are getting from external domains)
• Search & Traffic Volume (Relevant search engines searches and historical traffic on your webpages)

I would recommend look at your Link Structure and Freshness of your Site Content.

Thanks

Sushant