When Is Shared Hosting Ok And When To Move To Semi-dedicated Plans

From reading a good lot of posts here, I can see that HostGator seems to be a good company and that many have a good feedback to give about it. I've also been reading that you should avoid overselling, though understand that some companies, even though they offer "Unlimited space and bandwidth", they can manage it well. I guess HostGator falls in that category too. My questions here:

1) So what are the conditions under which it is okay to go for a company that offers unlimited space and bandwidth? (Expecting answers like 'if you just have a few blogs and don't require...')

I have read (here and elsewhere) that for most people's requirements, a shared web hosting would do. However, for someone a with a little more web baggage (individuals, not businesses), it is generally advised that they look at semi-dedicated hosting plans.

2) Based on the above quoted, objectively, what are the things that you should look at (like traffic per day etc) and what should be their values (for e.g. like 15000 uniques per day etc) while considering moving from sharing to semi-dedicated plans? I think some of them are the following, can you give me approximate values for these when I should consider the move?

a) Traffic
b) Number of active websites
c) Sites with heavy resource usage. (Can you give me some examples of this?)
d)...

What else am I missing in this list?