On the game's official page, BioWare has detailed what non-subscribers will be able to enjoy. They'll have full access to play through the story content and level their character up to 50. After that however, the limitations begin to set in. Players will have limited character creation options, be required to obtain a weekly pass to get in on Operations, be able to play three Warzone matches per week (unless they purchase a weekly pass), and more. To see the entire, depressing list of restrictions, head over to the SWTOR features page right here.
Does the free-to-play option have you interested in giving The Old Republic a look, or are all those restrictions deterring you from taking the plunge? From what I've heard, the story content alone may be worth checking out.