Having spent the last two years trying to make MongoDB work at large scale and only succeeding because I have a ton of resources (200+ db hosts), I can say it's more than just the spotlight.
Fair enough - I have no direct personal experience with Mongo myself. Coming from a more traditional DB background, what I've read indicates it has a lot of technological flaws, but I didn't want to be too harsh in my judgement without personal experience.
For my own curiosity, what issues caused you the most trouble? The lack of transactions combined with the extremely coarse grained write locking were what put me off the most.
Read performance compared to a lot of other offerings is a significant issue.
However, one of the biggest issue is the entire sharding design that is incredibly delicate and wouldn't pass even the most basic high availability requirements. I could go into a lot of detail about why it's bad but it would take too long.
After that, it's the hardcoded limitations that prevent true multi-datacentre sharding, the previously mentioned CPU issues with mongos, broken replica selection, inability to control primary/replica setting manually, etc.
It is a fundamentally broken product.