First and foremost, package recency.
If you run stable, which is released as snapshots ala Ubuntu, the packages are ancient.
If you run testing, which is a rolling distro ala Arch, they're a lot newer and pretty solid, but security updates lag.
If you run unstable, which is also rolling, things can (rarely) break.
Additionally, Ubuntu has decided to incorporate non-free software and drivers right into the base product, which gives a better out-of-the-box experience. In Debian this is all opt-in and requires a bit more effort.
Now, I run Debian testing on my laptop, and I'm a huge fan of the distribution, not the least because Debian is the bedrock on which at least a half a dozen other distros are built. But I can acknowledge that their more conservative approach to packaging does have its downsides.