Should I delete the file extensions of my URLS ? Matt Cutts pleads for his permanence. It is interesting that the user knows the type of file that is displayed in the query, whether it is a PDF or an HTML page, etc.
What factors determine the pagerank of a twitter page ? The same factors as any page. While in the particular case of Twitter comments on links and quality of those links. They do not look at the number of followers. At the time I rambled on this topic you can see it here.
The short urls are crawled (tiny urls) in the same way as the normal ones . Yes, they are tracked in the same way.
What impact does the server location have on Google positioning ? It is very important, depending on the IP it is known where the server is located and the results published before the same keyword depend.
Can my “blogroll” affect the reputation of my Blog on Google . Yes, if you link sites of bad reputation can affect the reputation of your Blog. If you also sell these links and find out you can have a good problem.