Hi Ryan,
In my opinion MODx will work as a good, very simple blogging platform, but not a great one. Niall intends to make a blogging site. I would think that if you used WP as much as you do MODx, it would be crystal clear.
Just ease of use is number one. First impressions matter, and Niall certainly didn’t have a good one. I’m certain that there are many like him, at least concerning blogging, and they’re just quiet about it. I’m speaking about Evolution, maybe Rev has a different scenario. The blog that comes with MODx installs is an assemblage of various parts, many of which, like Jot and Reflect, I still don’t understand. (My Ditto knowledge, hard-won through trying to follow documentation and making dozens of errors, is much better, but was also no cakewalk, though worth it in the end) Could I understand these snippets if I carefully studied them? Yes. But do I already have components for comments, tags, and the like in Wordpress already set up for me? Yes. How hard would it be to set up a MODx blog without having the pre-installed thing? Tricky and error-prone, in my opinion, from personal experience.
Some more things I like from the WP end... it’s like those annoying Apple ads... if I want social bookmarking on a page or post, and the ability to turn it off per page, there’s an app for that, with dozens of social site links and plenty of custom options. If I make a Post, I can categorize it on the fly and add tags, put in custom fields, allow or disallow comments and pingbacks, all built-in. When I save the posts, tags automagically go to the sidebar, etc. if desired, and auto-archives are updated. Those are just a few things, I’ve left out tons more. There are also many, many more plugins available for WP than there are for MODx.
You can do some of those things with MODx, but much more manual and set-up work is involved. But I’m not implying that MODx should do all of these. WP works so well with them because it’s structured with that in mind. To do this, I think MODx would need to be entirely re-structured, and its own "blank slate" approach is highly flexible and better than WP in other ways. I’ve even made a comparison chart of the strengths and weaknesses of each system, from my personal point of view. And yes, someone may say that if I’m complaining and want something else, build it myself, just so we have that covered. And yes, if someone really wants functionality of MODx but also wants a very rudimentary blog, it will certainly do for that.
The right tool for the right job. And I know that for many, if they have a fave system, they try to make it do absolutely everything. I don’t think that way at all.
Actually one last WP trump card - they offer free blogs on wordpress.com, no worries ever about keeping the system up to date, and so on, unless someone really wants more control, unlimited plugin usage, and maximum theming flexibility.
Maybe that’s more than you bargained for, but you asked.

And I’m ready to get shot down for having a dissenting opinion. That’s what forums are all about.