For free web hosting you should look for some features like:
A brand new hosting provider which boasts features that are too good to be true usually won't last for too long. They were offering 500MB of storage, 10GB of bandwidth per month, domain hosting, unlimited domain parking, CPanel with just about all possible features, absolutely no ads and this only for 150 posts in their forum.
If the site looks like it was bought from a man in a white van, or if it's using a design from a free template site, it's probably just a waste of time.
Advertising placed on your web space is how free web hosting providers support their service. Having someone else's ads displayed on your page is usually the price you have to pay for their free service. There are a select few hosts that won't plug in banners or other forms of advertising in exchange for your performing other services in their favor.
Forums, guest books, counters, polls are always nice features to add to your site. If you can't or don't want to handle installing such an addon from a third party you could look for a host that already provides it.
Also consider going with a host that will offer you a paid subscription if you go over this quota instead of simply blocking access to your page. On these basis you can choose a free web hosting.