Take servers, for instance. Some providers host web pages on their mail server. Our three web servers are dedicated to serving up your web pages when someone requests them. We don't put business web pages on our mail server. So if you are considering another hosting provider, ask them about this.
Some providers also try to do too much with the limited bandwidth they have.
There are some enterprising people who are hosting web pages on a single computer with a 28.8 modem. While their efforts are admirable, this is not really a place to host a serious web site. At MonadNet we have three T1 lines, for a total bandwidth of 4.6 Mbps.