Form filling, customs checks, export declarations, etc. More red tape and bureaucracy for our exporters to deal with resulting in increased costs to businesses which will be passed on to the consumer.
The extra hassle and delays for exporters trying to use the GB landbridge to transport goods to and from mainland Europe.
Any form of Brexit is bad for Ireland. This is being imposed on us against the wishes of everyone. The trade deal which has been negotiated is merely the least bad option. However the peace agreement and the all-island economy have been protected, and we have Michel Barnier to thank for that.
There are some the possible positives though, as you have mentioned. Every cloud and all that.