1. How important is a domain name to a business?
A domain name plays a critical role in Search Engine Results Page (SERP) ranking. The question is whether you want to establish your company name (brand value) or acquire a keyword which relates to the business that the companyinvolve.
2. Length of a domain name affects SEO/Pagerank?
Yes and No. As a rule of thumb - a short domain has more value - which inturn has some meaning to it. It could be an acronym or a word by itself. There are various domain names which are very popular that is extremely popular and they have better search ranking too (the reason because of the keywords that exist in the domain name). But what are my chances of winning and retaining the audience if I have a site like wearetheoriginalgoogle.com? So, it purely depens on what you want to do with the domain name, whether it is a throwaway or whether you are going to do something with that.
3. Sub-domains has value or not?
Yes and No again. I can create any and n number of sub-domains (amazon, msn, yahoo, google, business, etc.,) but it is based on the external links and the popularity among the internet world. mail.google.com is one good example, or del.icio.us : even delicious.com is available. So it does add weightage in some algorithms - provided it has audience in net world.
4. In What priority should onego for choosing adomain extension?
.com, .net, .org and then the rest. Each extension has a purpose - whether it is commercial or non-profit organization domain. But it is better to get all or most important extensions for your website, if you plan BIG.
5. Is it important to acquire related business domains to build traffic?
Not for a multi-billion dollar corporation, but for a small to mid-size, it is better to invest on keyphrase related domains as well to pull the traffic and to avoid competitors at later stages.
Cheers,
Shiva