Is there an app such as GoodReads for movies?
Yes, there is, Rinema.com      

Check out: Rinema Features

Feature Details

Movies: Add, Rate, Review, tag, link...

These little buttons, pretty much everywhere on the site, should come in handy to rate, review or mark a movie as "watched" or "to watch". And, while you review the movies you can also tag them (using the good old #tags) and link them to other movies by taggin them as done in the review below.
Review

To tag a movie, write the movie name in square brackets. So, [[The Shawshank Redemption]] will show as The Shawshank Redemption

Discuss...

Apart from using these little buttons to vote up/down and adding comments. You can also post updates through the homepage. And, you can tag movies, add #tags or tag your friends in them.

Oh! and you can see all conversations about a movie on the conversations tab on every movie page

Lists...

Hop over to the lists page &amp; check out some of the lists that others have created. Or better yet, create you own. It's easy peasy.

Search...

Use the search box on the top nav to search for any movie (by name), Celebrity or user.

If you are looking for a particular movie, but are unsure of the title or wish to search by cast/crew names or just want more options, hop over to the advanced search page (second last option in the dropdown menu)

Discover...

The Movies link in top nav will take you to our movie suggestions page where we show you some movies that you might want to check out. You can also refine suggestions by a particular genre and/or language.

The best suggestions though are always from people whose tastes you share. So, use the cinephiles link on the top nav to find interesting people to follow. Or invite your friends.

You can also toggle your updates feed on home page to show updates from all users or reviews(from all users). And, use that to find interesting people &amp; movies.

Collections...

You can add movies to various collections via this dropdown accessible from mymovies, individual and review modals.

we've have given three: Must Watch, Favourites, Guilty Pleasure to get you started with but, you can add as many new ones as you want. Just type the new name in the textbox and press Add. You can also delete collections using the [edit collections] link given in the collections list on the my movies page (towards the top).