I grew up with a mouse and keyboard as my primary controls for digital communication, so I can't text as fast as twenty-somethings or swipe (think Swype or Android's Gesture Typing) as quickly as teenagers. I spent my teens and twenties chatting on IRC, ICQ, AIM, Yahoo, and other chat networks, which is why I can type as quickly as I can.
A tablet is a good enough device for consuming content, but tap-typing or swiping are both very weak alternatives to a keyboard and mouse when it comes to producing content (e.g. forum posts, e-mails, Facebook posts, etc.) longer than 140 characters.
If you can find a tablet with a snap-in keyboard at a reasonable price, that may help bridge the gap in regards to communicating.
As for connection downstream, upstream, and stability, a cable or DSL line is likely going to offer a considerably smoother experience than 3G or any cellular data technology.
Tablet + 3G sounds serviceable in theory, but leave it to me to point out the faults in any plan, like a good editor should.
There's also the matter of cellular companies pricing mobile data like it runs on unicorn blood. Seriously, it's ridiculous. On a related, note, bandwidth caps are holding back innovation and America's downstream and upstream speeds are embarrassing when compared to other first world nations.
</rant>
When it comes to streaming video, 4GB per month probably doesn't go very far. The lowest cable or DSL cap I've read about in the US is 150GB and I think that information is at least a year old.
The good news is that a tablet would be excellent for off-screen viewing (e.g. someone else is watching the television in the living room and you want to watch an episode of something on Netflix on your tablet) and the price point is actually reasonable, especially for 7-inch tablets like the Nexus 7 2013.
But for typing e-mails and Facebook posts? Not so much.
Then again, as you can tell from the length of this post (and many, many others like it that I've posted), I do not represent the average user.
EDIT: Also, Comcast and AT&T are both horrible, horrible companies. (Unfortunately, AT&T U-Verse is the lesser of two evils in my area as politics has prevented any real competition. The U-Verse service is solid, but the U-Verse billing department is an absolute mess. I spent between 6 and 10 hours on the phone trying to get AT&T to honor the promo they promised when I first signed up.) I'd look for alternatives to those options in your area, but I understand that there may be no other viable choices (and believe me, I sympathize).
