First up I'd like to establish that I really enjoyed it on the whole. Larian did a great job and made something that has plenty of rough edges but also a ton of charm, character, individuality and ambition. Fusing so many disparate styles (mostly) successfully was a great achievement. If they ever do DLC an expansion or a sequel I'll be interested from the start.
That said here's some nitpicks, maybe if I should have given this feedback in the beta but I wasn't able to spend that much time in the campaign, partly down to some instability and save issues.
By the middle of the campaign I'd mostly been able to research everything. Holding large amounts of the map makes it way too easy to get huge amounts of research. If this isn't tweaked in some other way there should be some kind of limits or diminishing returns to the amount of research you can gain or some very expensive research items so you don't end up sitting on huge piles of research to no end. Also getting imp favour kept giving me small boosts to research which just felt weird since by that point I'd long since researched everything I could possibly want.
Winning the campaign by conquering the capitals is problematic. The defensive boosts are so great, the enemy so fond of turtling and the capacity to just stack huge numbers of troops on one tile means that it's best to conquer the whole map and then go for the capital. When you've captured 75% of the map though there's really no losing. This also feeds the issues with research above.
In the loading screen it says fortifications are affected by the proximity of enemy armies on adjacent tiles. It doesn't say HOW though. Does keeping an army next to an enemy capital reduce fortifications (like a siege) or increase them (giving them chance to reinforce)? Either way I haven't noticed the impact in gameplay.
There isn't any visible mechanic that accounts for the fact you can just sit in chapter 1 gaining resources for chapter 2 and so on. The whole chaptering system is pretty weird actually so I'm hoping Custom Campaign doesn't use it and you just don't have a chapter 2/3.
Some of the maps gave massive advantages to the defender in the RTS portion. Mechanically I quite liked this but it wasn't obvious in the campaign when this was going to happen and I don't think it was represented in auto resolve.
The buildings were a bit weird. The Tavern and Wizards Tower were pretty clear (though the Wizards tower was rubbish) but I was never clear which cards the others provided. It would also have been nice if I could build fortifications rather than have them jut sort of happen. This might have been a good place to sort research out as well, rather than just having regions provide research they could have just provided more spaces to build research buildings. If the research buildings were powerful enough this would add an extra dynamic to the campaign map as well as intrinsically add an element of diminishing returns.
The narrative elements were good but I wish there were a lot more of them. I don't know how expensive it would have been to add a lot of unvoiced exposition and chatter but it would have added a lot to the game.
Generals seemed to be consistently overpriced compared to their impact on the field.
In the preview I heard mention there were different dragon types for people who were more interested in the RTS portion or the arcade portion of the game. This didn't seem to materialise which is a shame. It might have caused some balance issues to have a dragon that was 75% more resilient and regenerated energy 33% faster at a cost of making your troops 25% less effective but I think the single player could handle a little imbalance.
I've realised I have almost nothing bad to say about the RTS elements, it's not perfect but it did the job pretty well with no glaring issues. Except Warlocks. I hate those guys. Larian did an amazing job of cleaning this up over the beta.
Wow, that was a lot of text. I guess it speaks to how much the game gripped me. Looking forward to future releases and (fingers crossed) mod tools. Either way I'm substantially happier I backed Original Sin this is the first Larian game I've seriously played and it gives a good impression.