For past express editions, I've been able to find a "compare editions" page that shows what features are available in express, professional, ultimate, etc. Something like this.
However, every comparison table I've seen so far omits the express editions. Surely Microsoft must have a list of things you get by upgrading to Visual Studio Professional from an Express Edition--after all, it's a free product, and they would like you to upgrade to a paid product! So, where have they hidden this feature comparison table, or "reasons to upgrade to Professional" list?
As far as I know there is no single table with a summary.
It depends on which version of express your talking about. Express is split up into a lot of different SKUs based on the type of product you're building; Web, Phone etc. Which is probably why there isn't an entry on this table for Express. 2013 was a point release to 2012 and there is a comparison of Express Web 2012 to Pro on MSDN.
The biggest single difference is that you cannot run plugins in Express. This means no Resharper, Test Driven .NET etc. This also applies to some plugins that you might consider to be written my Microsoft because they were not part of the official Microsoft release, for example plugins written my various other groups. This is a long-standing gripe for users of Express and looks like it is still the case for 2013, How to install Nunit plugin in Visual Studio 2013 Express?.