I can only give you my opinion as I'm not involved with MongoDB, I'm not a lawyer and I'm not an expert on AGPL. However, I spend a fair bit of my spare time reading licenses and thinking about them.

There are really 2 issues you need to keep in mind: 1) there is a difference of opinion about whether linking to an unmodified, GPL or AGPL licensed library is considered to be a "modification" of that library; 2) GPL and AGPL are different in terms of software that is only accessed via a network (i.e., the user does not receive a copy of the executable code).

In the first point, the FSF (who maintains these licenses) believes that linking to a GPL or AGPL license creates a "combined work" that requires a license that is compatible with the GPL or AGPL. In other words, you can't "convey" (distribute) the combined program unless the overall license has compatible terms. Other people disagree. To my knowledge, this point has not been tested in court. While the GPL itself is well tested in court now, that is for applications that are clearly based on a GPL licensed piece of code. Whether or not a library that is providing a utility function has that same legal protection is still untested (as far as I know).

However, there are a couple of things that I think are important about this. First, the intent of the licenses is clear. It's a completely jerk move to use a GPL or AGPL licensed library without considering the intent of the authors that chose that license. You might get away with it legally, but why be a jerk just to save a few bucks? (Not that it stops people...) Secondly, the intent of the license is clear, if you want people to use the software in a different way, please pick a different license! There are many appropriate licenses (and probably the LGPL is what a project like MongoDB should be licensed as). I'll talk a bit about that at the end.

With the second point, it's important to understand the difference in use cases for the GPL and AGPL. The GPL is intended for applications that are meant to be "conveyed" (distributed) to the user. The user then runs the program themselves. With a GPL or similar license, they enjoy the 4 freedoms of being able to run the software for any purpose, inspect the source code, modify the source code, and to distribute their modifications if they wish (as long as they grant the 4 freedoms to their users).

The AGPL is designed to give similar freedoms to people who do not receive an executable -- their only interact with the software is through the network. The only reason for choosing the AGPL is to ensure that users can receive the source code to: run the code for any purpose (rather than just the purpose the service provider allows), inspect the source code, modify the source code, and distribute their changes.

I don't know the history of MongoDB, so I don't know why they chose the AGPL. It is frankly a bizarre choice, IMHO. My only guess is that they intended that users of an online service be given access to the MongoDB source code. Why they thought that was important, I really don't know. The MongoDB developers are pretty adamant that the license does not restrict people from building services that are not AGPL.

In practical terms, for the moment anyway, it's not a big deal. Even if the AGPL applies to the combined software (and FWIW, I believe that it does), the only people who have standing to sue are the copyright holders of MongoDB. They have clearly said that they are happy with people not offering the 4 freedoms when making services using MongoDB. In other words, you can be relatively sure they aren't going to sue you -- and nobody else has standing to do so.

On the other hand, it's still a legal liability. If MongoDB changes hands some day and the new copyright holder has a different opinion, will you end up getting sued? Although I think it is incredibly unlikely, it could definitely happen. Crazier things have happened.

To sum up, AGPL is not problematic at all, if you are using it as intended (as an aside, I actually don't like some of the wording, which I think is a bit hand-wavy at times, but it's the best license that I know of for the niche that it occupies). The only problem is that MongoDB developers are using it in a way that differs from its intended purpose and are thereby muddying the waters in terms of communicating what they want. I suspect they simply made a mistake originally and now it's just too difficult to make the license change.