Perl: 203 202 201 198 195 208 203 / 231 chars
$/=\0;/^(the|and|of|to|.|i[tns]|or)$/i||$x{lc$_}++for<>=~/[a-z]+/gi;map{$z=$x{$_};$y||{$y=(76-y///c)/$z}&&warn" "."_"x($z*$y)."\n";printf"|%.78s\n","_"x($z*$y)."| $_"}(sort{$x{$b}<=>$x{$a}}keys%x)[0..21]
Alternate, full implementation including indicated behaviour (global bar-squishing) for the pathological case in which the secondary word is both popular and long enough to combine to over 80 chars (this implementation is 231 chars):
$/=\0;/^(the|and|of|to|.|i[tns]|or)$/i||$x{lc$_}++for<>=~/[a-z]+/gi;@e=(sort{$x{$b}<=>$x{$a}}keys%x)[0..21];for(@e){$p=(76-y///c)/$x{$_};($y&&$p>$y)||($y=$p)}warn" "."_"x($x{$e[0]}*$y)."\n";for(@e){warn"|"."_"x($x{$_}*$y)."| $_\n"}
The specification didn't state anywhere that this had to go to STDOUT, so I used perl's warn() instead of print - four characters saved there. Used map instead of foreach, but I feel like there could still be some more savings in the split(join()). Still, got it down to 203 - might sleep on it. At least Perl's now under the "shell, grep, tr, grep, sort, uniq, sort, head, perl" char count for now ;)
PS: Reddit says "Hi" ;)
Update: Removed join() in favour of assignment and implicit scalar conversion join. Down to 202. Also please note I have taken advantage of the optional "ignore 1-letter words" rule to shave 2 characters off, so bear in mind the frequency count will reflect this.
Update 2: Swapped out assignment and implicit join for killing $/ to get the file in one gulp using <> in the first place. Same size, but nastier. Swapped out if(!$y){} for $y||{}&&, saved 1 more char => 201.
Update 3: Took control of lowercasing early (lc<>) by moving lc out of the map block - Swapped out both regexes to no longer use /i option, as no longer needed. Swapped explicit conditional x?y:z construct for traditional perlgolf || implicit conditional construct - /^...$/i?1:$x{$}++ for /^...$/||$x{$}++ Saved three characters! => 198, broke the 200 barrier. Might sleep soon... perhaps.
Update 4: Sleep deprivation has made me insane. Well. More insane. Figuring that this only has to parse normal happy text files, I made it give up if it hits a null. Saved two characters. Replaced "length" with the 1-char shorter (and much more golfish) y///c - you hear me, GolfScript?? I'm coming for you!!! sob
Update 5: Sleep dep made me forget about the 22row limit and subsequent-line limiting. Back up to 208 with those handled. Not too bad, 13 characters to handle it isn't the end of the world. Played around with perl's regex inline eval, but having trouble getting it to both work and save chars... lol. Updated the example to match current output.
Update 6: Removed unneeded braces protecting (...)for, since the syntactic candy ++ allows shoving it up against the for happily. Thanks to input from Chas. Owens (reminding my tired brain), got the character class i[tns] solution in there. Back down to 203.
Update 7: Added second piece of work, full implementation of specs (including the full bar-squishing behaviour for secondary long-words, instead of truncation which most people are doing, based on the original spec without the pathological example case)
Examples:
_________________________________________________________________________
|_________________________________________________________________________| she
|_______________________________________________________________| you
|____________________________________________________________| said
|_____________________________________________________| alice
|_______________________________________________| was
|___________________________________________| that
|____________________________________| as
|________________________________| her
|_____________________________| with
|_____________________________| at
|__________________________| on
|__________________________| all
|_______________________| this
|_______________________| for
|_______________________| had
|_______________________| but
|______________________| be
|_____________________| not
|____________________| they
|____________________| so
|___________________| very
|__________________| what
Alternative implementation in pathological case example:
_______________________________________________________________
|_______________________________________________________________| she
|_______________________________________________________| superlongstringstring
|____________________________________________________| said
|______________________________________________| alice
|________________________________________| was
|_____________________________________| that
|_______________________________| as
|____________________________| her
|_________________________| with
|_________________________| at
|_______________________| on
|______________________| all
|____________________| this
|____________________| for
|____________________| had
|____________________| but
|___________________| be
|__________________| not
|_________________| they
|_________________| so
|________________| very
|________________| what