Bought my Mom a Roku 2 XD for Christmas, and you need the expansion slot if you download a ton of channels, download any of the games (angry birds), and not really sure what else it might be needed for. The Roku has rather limited memory, and each channel/game uses up that memory.
As far as channels go, most of them were complete crap. I got her the Plex Media private channel, so she could stream all the HD tv shows/movies I constantly download off the net and keep stored on my pc. The only channels she herself, a 64yo woman, actually had use for was Jewelry TV, and a bundle channel of QVC, QVC UK, HSN & HSN2.
There were a few cheesy channels that show really, really old movies/tv series, a few channels that show bootleg movies (one had been running a really bad cam of Captain America back in January). The website itself lists most of the channels you can get, and googling "roku private channels" will also bring up a few lists to peruse.
We have Netflix, but she doesn't really use it, so it's on the Roku, she just never uses it.
Honestly, there are loads of channels, but for me it's 99% garbage. Loads of religious channels, news channels, info channels, but for pure entertainment media it's all kind of public domain type crap. Other than Netflix/Hulu/Plex I'd have no use for it.
And now that she's gotten used to using Plex, all I ever hear is "I missed my shows last night, can you download them for me?" LOL.