The key difference is the GPL requires code modifications to be open-sourced if the code is distributed in any way, while the MIT does not.
If I were to take a piece of open source software that is licensed under the GPL (like WordPress), any code I add that builds on top of it (pretty much all PHP that plugs-in to WordPress) must be licensed under the GPL as well. If I combine MIT licensed code with a GPL product, the new, whole product must be licensed under the GPL. In this way, the MIT is compatible with the GPL. However, MIT software distributed with GPL software (like WordPress) is no longer MIT licensed but becomes GPL licensed.
It is possible combine MIT licensed code with GPL code and retain the MIT license for the MIT part only if it's an independent product. The SimplePie project for example (which is actually BSD licensed -- similar to MIT) can continue to be BSD licensed even though it's part of WordPress because it's independent of WordPress. (Keep in mind that you do not have as many rights as the owners of the WordPress copyright so I'm just using this as an example.)
The question here is whether the plugin can be considered a separate work or if it's merely a modification of WordPress. This is a legal question best left to a qualified attorney. That being said, the general feeling of the WordPress community is that the PHP code of plugins must be distributed under the GPL. This is evidenced by all major commercial plugins being licensed under the GPL.
There is an excellent theme-specific article on this topic which may help shed light on this topic.
Keep in mind also, that if you use the MIT license, anyone may take your code and release it under the GPL. They may also take your code and distribute it with their modifications without providing source code turning your work into a part of their proprietary product. This is very much counter to the goals of the GPL.
I suppose it would be possible for some very limited plugins to be licensed under the MIT without much room for debate (a CSS-only plugin perhaps). But in general, there is legal risk to using MIT while there is much less to using GPL.