Is using a relational DB for storing votes and (accessed resources) overkill?
In general, clustered-NoSQL databases offer you better horizontal scalability. So, as you scale you can get higher capacity (both memory &amp; compute power) by simply adding new nodes. So, it will be a good insurance for the future.

When it comes to ACID properties, NoSQL databases offer flexibility. So, it boils down to how much ACIDness you want and how much performance you are willing to give up for it. Seems that you do not need very high level of ACID. I am sure you can get good performance in lieu of that. 

PS: I do not want to comment specifically about MongoDB. You should do your research and choose the NoSQL db that fits your performance &amp; functional needs.