Query Plans Change Performance Worse in SQL Server 2014
This is not an issue, it is by design and is considered a performance improvement. 

If your queries do not run as expected you will need to address this in the code of your database/application. 

You can force the old carnality estimator by using the trace flag 9481 at the server, session, or query level (using OPTION (QUERYTRACEON 9481)). This will force it no matter what the compatibility level of the database is.

There is also a ton of information that the CSS Team posted here.

A more detailed post on the Cardanility Estimator and changes around that process in SQL Server 2014, includes example.