There are technical limitations as others have noted. The attitude and focus, though, I think is what kept OpenBSD in their own niche.
The only technical limitation I've run into as far as using it as a daily driver OS, is that it can be a bit slow even on very fast hardware. It's simply not optimized for desktop use, though it is certainly usable with a few tweaks. It's great for older hardware too, where the speed issue is relative. In fact, I've found it to be a perfect fit for a couple of old laptops in my collection.
Harder to say whether it's easier to bring OpenBSD to where competition is on these or to bring competition's code to where OpenBSD is, esp as some safety/security tech makes that less relevant. Hard to say...
Regardless, it's a high-quality UNIX that's worth having around. I'm glad to see them improving in areas where it's weaker while not throwing away strengths like some projects would.
I recently inherited an old-ish laptop (I don't know the exact year it was built, but it has a Windows Vista sticker on it, so 2007-2009, I guess) and decided to install OpenBSD on it, mainly because I have always wanted to give it a try on real hardware as opposed to a VM.
I was pleasantly surprised to find that all the pieces I care about (sound, video, wifi, ethernet, ...) Just Worked(tm). That was nice. More heavyweight applications like Firefox or Clementine slow the system down substantially, but otherwise it runs nicely.
Systems in 2007 had no problem running Firefox. I'm surprised to hear it has a significant impact.
Maybe Firefox has become much more resource intensive? That would surprise me because processors haven't gotten much faster, Mozilla has focused on reducing memory consumption and improving responsiveness, and because of the rise of mobile, a resource-constrained environment.
Matter of fact, I can prove it with an example [1] that predates OpenBSD's approach while succeeding in market at least while under good management (sighs). Used a combo of good design, OS features, and code quality to achieve reliability most still haven't beaten. Coding approach, which I'm focusing on here, was to alternate between developing features and fixing bugs with tests (esp regression) run on weekends while developers were off. One week build stuff, tests on weekend, one week fixing by priority, rinse, repeat. That simple. OpenBSD took it further with security focus and systematic audits but this basic method produced robust and marketable code. So, what's other commercial players' excuse? ;)
[1] http://www.itec.suny.edu/scsys/vms/ovmsdoc073/ovms_archived/...
I've heard this, but what exactly is missing? Do you mean features such as a hypervisor?