Of course "bad" is a noun:
1993 Dog World Nov. 28/1 It is a relatively in-depth look at both the good and the bad in commercial canine nutrition.

Actually, MWDEU doesn't say it's common, it says "sometimes". But I think it's a normal expression round my way.

It's common in speech and speechlike writing, according to Merriam-Webster's Dictionary of English Usage.

Now Margaret's curse is fall'n upon our heads,
When she exclaim'd on Hastings, you, and I,
For standing by when Richard stabb'd her son.

Shakespeare, The Tragedy of King Richard the Third, III iii

Yes, if you're going to discuss etymology, then yes, knowing some historical linguistics would certainly help.

Gallitrot, I'm not sure what you mean about "mother tongue only has worth if viewed through the learning of another" - I never said anything about that.

And yes, spelling can be affected by pronunciation. We have evidence to show that the pronunciation of words like "forehead" and "waistcoat" changed because of the spelling. But we have (I think) no evidence that this happened in the middle ages, when people spelled how they pronounced, and not the other way around. All the examples of Norman influenced spelling change I am aware of did not change the pronunciation.

Good grief, I never said that 200 years of lingustic studies was unquestionable fact. However, we have a prevailing theory, which explains a lot, is testable, and lets us make predictions which have been fulfilled (Saussure's coefficients sonantiques is a good example). Language study is a science, and like a science, if you have a problem with the prevailing theory, then do the work and come up with a better theory, one that explains everything the current theory explains and more.

Brus, however past participles might behave in Latin is irrelevant to English.

MWDEU on "very" with past participles: http://books.google.ca/books?id=2yJusP0vrdgC&lpg=PP1&dq=merriam-websters+dictionary+of+english+usage&pg=PA938&redir_esc=y#v=onepage&q=merriam-websters%20dictionary%20of%20english%20usage&f=false

MWDEU on participles as adjectives: http://books.google.ca/books?id=2yJusP0vrdgC&lpg=PP1&dq=merriam-websters+dictionary+of+english+usage&pg=PA717&redir_esc=y#v=onepage&q=merriam-websters%20dictionary%20of%20english%20usage&f=false

The grammarian Quirk has criteria we can use to tell if a participle has adjective status.

attributive use: She gave me an annoyed look.

predicative use with "seem": She seems rather annoyed.

premodification with "very": She’s very annoyed.

comparison: She’s growing more/less annoyed by the minute.

etc… Please look at the links above.

Quirk claimed that modification by “very” is “explicit indication” that a participle has achieved adjective status.

“Yu can't say that belcose is beclysan, influence'd by French, but then say that close has nothing to do with clysan. That's a "non-sequitur".”

The way I interpret the OED’s etymology of “beclose” is that it is a continuation of “beclysan”, but the second element was replaced by “close”, which was borrowed from Old French. 

I have a background in linguistics. You say it's “all guesswork”, but isn’t. Historical linguistics is not just guesswork. You're making speculations that conform with what you want to be true - that's guesswork. But I'm looking at the theories that have been formed over the past 100-200 years based on observation and rigorous methodology. Sure, I could be wrong, but I'm much more likely to be right. If you think the entirety of historical linguistics is wrong... well you've got quite a job ahead of you.

Yes, the Norman influence respelled a lot of words. "gilt" became "guilt", "mȳs" became "mice". But the respelling never changed the pronunciation. You seem to be suggesting that with "close", the spelling changed the pronunciation. This is a big deal, and you need evidence.

OE "clȳsan" probably had a long /ȳ/ - a high front rounded vowel, like modern French "tu". In ME it unrounded to /i/ - as in modern English "heat". You've provided no evidence that it had a different vowel sound, or that our modern word derives from a certain dialect variation. Sound change is regular. You can't just make stuff up.

"Anent close, I think it might help if we note ü insted of y and üü insted of ȳ … and uu for ū. Thus clüs, and cluus are not far from the Latin clusa and OHG klúsa. I'll leav it to Ængelfolc as to whether the P-GMC word came from Latin or a common PIE root."

It might help if we use different phonetic symbols? Using different phonetic symbols doesn't make a sound change more likely. And the Proto-Germanic word didn't come from the same PIE root as the Latin word. We know this because both words begin with /k/, and Latin /k/ corresponds to Proto-Germanic /h/.

We are reasonably certain that "clūs" was borrowed from Latin "clūsa", and that OE "clūs" became "clȳsan" with a fronting of the vowel.

"The staff 'y' has an utterly nother sound in ME than in OE … So what is a scribe to do? Now, yu think that 'oo' in ME isn't the 'oo' as in loop. It's either that or a looong 'o' so if someone wrote cloos … that would with a slightly longer 'o' sound … which, if said quickly, sounds a lot like ü … either way, it isn't the same as close and likely from the OE clüs or clus … but near enuff for writing."

This is all speculation. The fact that a long "o" might sound a lot like ü if said quickly is irrevelent. They were still presumably separate phonemes, and if you're saying that a specific sound change happened here, you need good evidence.

munec > monk
sum > some
hearsum > hearsome
tung(e) > tong > tongue
wundor > wonder

First of all, the spelling of these words is well understood. We don't have to resort to saying "it was really chaotic, there were a lot of dialects, anything could have happened."

Second of all, these words all have short "u" so they're not relevent to the question of "close".

Third of all, in these words, the "u" was changed to "o" purely for ease of reading. In the calligraphy, "u" looked like two vertical strokes (minims), and "m", "n" and "w" also looked like a series of minims, so a combination of these letters was hard to read. So the scribes changed "u" to "o" in these words. *The pronunciation did not change as a consequence of the spelling.*

þurh (thurh) > through

OE "þurh" had a short vowel. As I understand it, the loss of the final fricative lengthened the vowel, which was then spelled "ou" - "ou" being a Norman convention for spelling long /ū/. *The pronunciation did not change as a consequence of the spelling.*

"Take a look at the words dūstig, dystig, dȳstig (all for dusty; y=ȳ=ū) … dust itself is dust and dūst (u=ū). If we were to put how we now think each of those vowels sound then we would come up with some pretty wide sundernesses among them. Dialects? Accents? Why did the scribes choose the spellings they chose? Why do we now mark some of the vowels with the ¯ for the same word? So you see, I don't hav the same trust that some of the words hav been rightly markt in the first place. I take it all with a grain of salt."

So your reason for being skeptical of the whole enterprise of historical linguistics is one word? According to Upward and Davidson's A History of English Spelling, "dusty" from "dystig" is a West Saxon respelling. Where we find surprising results, we can often attribute them to dialect variation. But that doesn't mean sound change isn't regular. If you think that "clȳsan" came to be pronounced "clūs" because of some dialectal variation, ok - which dialect? Where is your evidence?

AnWulf,
The Wiktionary entry gives no references for its etymology of "beclose". This is what the OED says:

"Originally Old English beclýsan , < be- prefix 1 + clýsan : see cluse n.; subseq. changed to close n.1 after French."

Now you say "close" is a blend of OE and OF. Earlier you said that it was simply the spelling that changed. It is certainly possible that the presence of an English word with a similar shape might have made the borrowing of the French word easier. But this is not simply a spelling change. It can't be, because the English word was not pronounced with the same vowel as OF "close".

Thanks, porsche, you said it better than I could. I'll add that the test for mass nouns is: can you put a number in front of it? Take "furniture", a textbook mass noun. We don't say "one furniture, two furnitures", we have to use a counting word, like "one piece of furniture, two pieces of furniture". The same with "Lego".