First, you leave the office and start wandering Times Square without coat, scarf or gloves on the first truly frigid day of the season.  Then, between mouthing silent epithets at the ruthless producer (only in show business does the person who produces nothing of value end up with the title of Producer) with whom you just concluded a particularly unpleasant meeting (which prompted your hasty coatless winter stroll) and dodging the cheap thrill seeking hordes-- you find yourself wondering why there's a ruthless and no ruthful?  And knowing you won't  know till you get back to your desk, you amuse yourself (which also serves to distract you from  that little crackling noise  coming from the frozen nipples on your chest) by trying to think of other "less"  suffixed words  that pose a similar question, (which as it turns out, there're very few) and make your way back to the office with chattering teeth and words like reckless and feckless running through your head, until you're safely thawing out and embarking on your etymological mission. 

Ruthless.  
The word  is middle-English in origin, referring to the biblical story of Ruth*; the theme of the story being kindness, love, and redemption.  To say someone is ruthless is to literally say that that person is unlike Ruth; to wit: Not nice. And I found this little ditty:

Ruth rode on my motorbike

Directly back of me;
I hit a bump at 65,

And rode on Ruthlessly. 

Copy on movie poster reads: 

From pagan idolatry to the worship of one God...from untouchable priestess to woman immortal!

Wow, some days I'm just so proud to be an entertainment copywriter.
http://thegrammarexchange.infopop.cc/eve/forums/a/tpc/f/340600179/m/7936086063
* I guessed this during my walk.  I swear. 

Reckless

According to the Random House Unabridged Dictionary(1977), reck is a verb that means "to have care or concern (for)".  The adjective reckless is formed on that base.  

Then reck I not, when I have lost my life. --Chaucer. [1913 Webster]

I reck not though I end my life to-day. --Shak. [1913 Webster]

Of me she recks not, nor my vain desire. --M. Arnold. [1913 Webster]

Can't seem to find anything in the way of modern usage, seems like Reck just rode off  into the sunset while its antonymic sidekick stayed around ...I reckon. 

http://www.thefreedictionary.com/recklessness

http://definitions.dictionary.net/reck

Feckless. 
In the 1500′s, Feck appeared as a dialect word in Scotland and the north of England to mean “the purpose, drift, tenor, or substance of a statement.” It was also used to described “efficacy, efficiency, and value.” This is the obvious root meaning of the word feckless - being devoid of feck.

Feck can also be used as a slang verb for “to steal.” James Joyce uses it in Portrait of an Artist as Young Man; “They had fecked cash out of the rector’s room.” The similarity in sound between feck and fuck has also lead to it being used as a euphemism. It inflects in the same way fuck does, hence its value. The Urban Dictionary suggests that this meaning originated in Ireland although this may be due to the frequency of its use in the mid-90′s UK sit-com, Father Ted. I couldn’t find any sources to say when the euphemistic feck first appeared but it is clearly a phonetic derivation.
http://etyman.wordpress.com/2009/06/04/feckless-fɛkləs/

And in typical internet search fashion, I stumbled upon: 

Obama And Iran: Reckless Or Feckless?

http://www.theamericanconservative.com/dreher/obama-israel-iran-reckless-feckless/

 Regardless v. Irregardless

Irregardless is a word, but it's not a proper word.

In the immortal words of Douglas Adams:  Don't Panic.  Regardless or irregardless of how a word that is not a word can be a word, all you really need to know is that for all intents and purposes (as well as for the "experts" I consulted in last 5 minutes) there is no such word as irregardless because regardless already means without regard. The —ir prefix is redundant

The speculation among accessed references is that it may be a blend, or portmanteau word, of irrespective and regardless, and it just stuck and hung on (or is that hanged on? Hmmm, see below.)

http://grammar.quickanddirtytips.com/irregardless.aspx

http://en.wikipedia.org/wiki/Irregardless

LEFTOVERS FROM STUFF FOUND ON SIDE OF THE ROAD 

DURING ABOVE MISSION

Hanged vs. Hung.
Simple.  People are hanged, curtains are hung.  Oh, wait a minute, not so simple, cause I don't believe you fell off your step ladder while you were hunging your curtains. I'll leave it up to you to find out more, I'm hanging up on this one.

Sneaked vs. Snuck
Sometimes, two forms of a word may be correct. For example, both sneaked and snuck may be used as the past and perfect tenses for sneak.Example: She snuck up on him. OR She sneaked up on him.Example: She has snuck up on him twice while he was napping. OR She has sneaked up on him twice while he was napping. Snuck” is still considered slightly less than acceptable, but according to Harvard researchers (can't argue with them) , 1% of the English-speaking population switches from “sneaked” to “snuck” every year, with the shift being most powerful in America. How they came up with that data is probably the more interesting story:  "Hello, Mr. Drifter, I'm calling today to ask you a few questions about your use of sneaked and snuck...did I catch you at a good time?" 

Lighted vs. Lit
Sometime after 1800, people began to prefer the irregular verb “lit” to the regular past tense “lighted”. “Lit and “lighted” both currently exist as fully acceptable past-tense forms of “to light.” 

Burned vs. Burnt
First, verbs don’t always evolve at the same rate in different countries.  Also, though it seems like nobody knows why, British English speakers have held on to irregular verbs more than American English speakers, which is why they say “dreamt,” “burnt,” and “learnt” in Britain, and we say “dreamed,” “burned,” and “learned” in America.

Ellen reminded me this morning to include:  Flammable v. Inflammable.

They are both proper words and mean the same thing.  And were both used interchangeably until the 1920s when the National Fire Protection Association promoted usage of flammable because of a concern that people would interpret the "in"* in inflammable to mean "not".  But they were only marginally successful and the two words continue to duke it out today.

*Actually, the -in, in inflammable was derived from the Latin preposition meaning en-(like enflamed) and not the Latin prefix meaning un.

http://chemistry.about.com/b/2011/01/08/flammable-versus-inflammable-what-is-the-difference.htm

Other References of note

 “English Irregular Verbs,” Wikipedia.http://en.wikipedia.org/wiki/English_irregular_verbs
 McWhorter, J. Our Magnificent Bastard Tongue, Gotham Books, 2008.
 Stephen Pinker “The Irregular Verbs,”http://pinker.wjh.harvard.edu/articles/media/2000_03_landfall.html
 Gellene, D.  “How English adds the '-ed'” Los Angeles Times,  October 11, 2007. http://articles.latimes.com/2007/oct/11/science/sci-verbs11
Yong, E. “The Culture Genome: Google Books Reveals Traces of Fame, Censorship, and Changing Languages,” Discover Magazine.http://blogs.discovermagazine.com/notrocketscience/2010/12/16/the-cultural-genome-google-books-reveals-traces-of-fame-censorship-and-changing-languages/