Currently that's the only reason, and only because PAE isn't worth the hassle to enable. You have to run a 32bit browser anyway if you care about having Flash. I'm sure there are other gotchas which might bite you in 64-bit as well.
Flash is junk, but besides that, there is a 64-bit flash available. And you can run 32-bit browser plugins with a 64-bit browser anyway (nspluginwrapper)
Lulz. I remember how broken Gentoo used to be, and that was years ago when it was popular. Having been at this for 15 years, I'll take "works" over feeling awesome about pages of compiler output. And I'd certainly recommend Ubuntu for someone coming over from OSX for the first time.
There's where you're wrong
I've been on gentoo for 5+ years (along with numerous other distributions before and during). I must say it isn't broken at all. I love the ability to get any package i want, live ebuilds. kde svn with qt 4.5_beta1 is very nice for me.
I run a crazy system too (~ARCH, with numerous live packages, my custom kernel, zen-sources). I also know someone who has a gentoo box with an uptime of almost a year, and it would have been longer than that probably except for a power outage.
Ubuntu with kde-nightly for me is blech compared to gentoo. It's impossible to get all the packages i want for it. So i have to install kdelibs 4.1 just to get some packages i want.
my second favorite distro is arch linux, but that's just me. I find it MUCH better than ubuntu. If you're not into the compiling thing, but arch will still give you that option (compiling isn't a big deal for me at all since i have a dual core anyway) the install of the system, X, and kde took maybe 8 hours total, unattended mostly. And updating is a single command
Find me another distro that lets me build packages WITH or WITHOUT debug symbols as i please (for live packages i don't strip them), and one that lets me build packages with native and that gives me qt 4.5_beta (a must for webkit-kpart/konqueror) and kde 4.2/nightly