Well it will heat up more easily and be more likely to throttle back because of it. But you'll have to be doing something fairly intense, the coooling is much better than it was. That is the trade of for using a closed laptop as a desktop I'm afraid.
There is a way to keep the screen turned off (but open) when connected to an external display as well. I do this myself, only because I've noticed it stays cooler when open. Here is a link on how to do it.