My issues with the caighdeán are:
* The initial mutations after de and do. In my opinion, after the combinations den and don, the initial s- should always take the t- if applicable. Thus, I prefer don tsagart, don tsaor, to don sagart, don saor. As far as I know (but I am not even sure), the Caighdeán tells us not to use the t- if the noun is masculine. Thus, we should say don saor but don tsióg, for instance. I don’t think this is necessary or feels natural. I write don tsaor, don tsióg, don tsaor bheag, don tsióg bheag.
* The use of the present and future forms ending in broad -s in direct relative clauses. They are not part of the caighdeán, but they are widespread in dialects and, moreover, make it easier to distinguish the two kinds of relative clauses, so they are didactically a good thing. I nowadays try to use them consistently.
* I would also like to reinstate ins an, ins na instead of sa, sna (although I must say that I don’t use the longer forms myself). This is again about didactics. Many learners struggle with sa, sna, not understanding their exact relationship with i, in. I think the relationship would be easier to explain with ins an, ins na.
* I would not revive all dative forms, but I would prefer to see the dative forms of feminine verbal nouns ending in -ach used wherever the verbal noun is preceded by ag or a = do: thus, caismearnach – ag caismearnaigh, leimneach – ag léimnigh, and so on. This is purely a question of personal taste, I am afraid.