Expression Engine is known for being extremely easy to theme with. Joomla is not that difficult in my experience, but it lacks in terms of forum solutions and so many other areas in my opinion.
Expression Engine uses its own template language which avoids any form of language such as php, asp, cold fusion, java etc.. its all done with a tagging system. Also templates are stored in the database and not in separate files so you don't need
FTP to make changes to your theme / layout.
I've not actually created a template for EE myself so I can't say how difficult it is, but I will say it appears to be very easy looking at their documentation and screen casts.
EE does come at a cost though, commercial license is more expensive than private. You can also get a very simplified version of EE called EE Core and you can play with its features in there and test templating etc..
EE also has a shopping cart module which supports PayPal etc..
EE will not require major hacks, rather the forum element integrates seamlessly for a hassle free website.
Movable Type may also provide alot of these features, but I can't comment on this system as I have very little knowledge of it - although it does look great also.
Of course if you need a FREE system then you are limited in choices.