One thing I got nagged about upgrading was that it would be less secure. But the joke here is that this forum has numerous modifications to it's code, and for each modification there's a more likely chance of a crack being opened. And see, if we installed a modification (which is made by a third-parties, not the actual phpBB team) and someone found a crack, we wouldn't know about since it's unsupported by the phpBB developers anyway. So really, there's probably bugs all over that we don't even know about.
But all the modifications(which can open cracks) for new features we make on this forum (phpBB 2), are actually standard features in phpBB 3. Thus, those features are supported by the developers of phpBB, so if there was a bug with them, a fix would be included in an upgrade for the forum and would get fixed rather than having bugs sitting around that are unsupported and won't get fixed (because they'd be unknown, by the way).
Some of the modifications are like the birthday modification, or the gender modification. Or even spoiler tags, which didn't come with phpBB 2, so it had to be modified to be able to. And in phpBB 3, the birthday and gender fields are actually built-in features, thus we don't have to modify it.
And as far as bugs in phpBB 3 itself, it's actually been going through bug fixing and public testing for over a year and has finally be released as a gold version. But if any bugs in phpBB 2 popup, they won't be fixed because phpBB 2 is no longer supported.
Avatar original image by "sometimesong" of Flickr, released under the Creative Commons license (share-alike, attribution, non-commercial, no-derivatives)