Following article has nice information focusing on multiple factors conferring the tropism to virus...http://www.ncbi.nlm.nih.gov/books/NBK8149/

http://www.nature.com/nri/journal/v9/n9/fig_tab/nri2623_T1.html

Also this content has relevancy to your query;

As parasitic entities, viruses have to compromise with numbers of positive and negative factors present in target cells for their survival. In the absence of an appropriate interaction with cells, they do not replicate at all. Viral tropism therefore can be determined at each replication step, beginning with the entry into cells and ending with the progeny production from cells. There are two major types of viral tropism, that is, the receptor-dependent and -independent tropisms.The binding of viruses to host cells is the first step in determining tropism and pathogenicity. 

Following article has nice information focusing on multiple factors conferring the tropism to virus...http://www.ncbi.nlm.nih.gov/books/NBK8149/

http://www.nature.com/nri/journal/v9/n9/fig_tab/nri2623_T1.html

Also this content has relevancy to your query;

As parasitic entities, viruses have to compromise with numbers of positive and negative factors present in target cells for their survival. In the absence of an appropriate interaction with cells, they do not replicate at all. Viral tropism therefore can be determined at each replication step, beginning with the entry into cells and ending with the progeny production from cells. There are two major types of viral tropism, that is, the receptor-dependent and -independent tropisms.The binding of viruses to host cells is the first step in determining tropism and pathogenicity. 

Hi Jordi, Jawwad made a really good and complete answer with the list of the most common determinants of viral tropism, I would add to this list the presence of restriction factors (as example Trim5 for HIV) that could explain the tropism to some cell line or why the virus can't replicate or less pathogenic in some host species.

For a gven virus, the mode of entry may differ: fore example, cell-free virus involving receptors  vs. cell-to-cell transmission.  This issue can be important wrt vaccine development - for example, mucosal exposure (such as sexual) vs. parenteral.

Thank you all for your quick answers and your help. And thanks especially to Jawwad, for your very complete answer, and these references that will help me to solve many doubts. Best regards