there are many guidelines to composition, the rule of thirds is one, off-center is one, but guidelines can be broken, and sometimes when you go by the book and follow them to a T they will fail too.
what's more important is asking yourself very honestly, instead of being happy with results immediately - does this picture REALLY work? if so, why? if not, why not? this will give you a better understanding of what you are doing right and what you can do to improve this the next time you take a shot.
that's not to say that you shouldn't be happy with any picture you take - it's about maintaining a balance.. and of course if you think a picture is good, and more than one person says that it doesn't work for them, then take them seriously, give them the benefit of the doubt and find out why they think that way and see if you can see what they mean.
taking a look at many good pictures, or at least what is deemed good by the majority at the very least (since it's really hard to say that a picture is DEFINITELY good).. that will help too. take a look at the top galleries in pbase. take a look at the most interesting pictures in flickr. get hold of books of famous photographers, if you're interested in landscapes, just devour them, the library has loads of these actually. if you need help as to how to find these, let me know. i hope this helps.