This sentence is often given as an example of when it's acceptable to use a comma splice, that is to divide independent clauses with commas, rather than semicolons. I think it's mainly to do with the fact that each clause is very short and that the structures are similar.

http://www.utexas.edu/courses/sutherland/Punctuation.html

From Wikipedia - Strunk & White note that (comma) splices are sometimes acceptable when the clauses are short and alike in form, such as:

"The gate swung apart, the bridge fell, the portcullis was drawn up".

Incidentally, there's an interesting article on the possible waning of the semi-colon at The Australian - http://www.theaustralian.com.au/opinion/life-of-the-semicolon-heading-for-a-full-stop/story-e6frg6zo-1226109371840

In Michael Swan's Practical English Usage (Oxford 1995), which is almost my 'bible', he says that 'used to' can indeed be used as a modal auxiliary (ie without 'do'), especially in formal British English. Examples:

"I used not to like opera, but I do now"
"Used you to play football at school"

But he goes on to say that this use is rare. I certainly don't teach it to my students, and I can't remember seeing it in any of the (British) course books I use. Swan then says that in an informal style we normally use normal question and negative forms with auxiliary "do".

Advanced Grammar in Use (Cambridge 1999) says much the same; that these forms are used in formal spoken and written English, giving the examples:

"There used not to be so much traffic"
"Used you go to university with the Evans brothers"

So I wouldn't say that this use of "used to" is no longer used, but it is mainly to be found in more formal language. And my impression is that the formal negative form is used rather more than the formal question form. Raymond Murphy, in the Upper-intermediate version of 'Grammar in Use', gives the negative forms - "I didn't used to like him (or I used not to like him)", but gives no alternative to "Did you use to eat a lot of sweets... ?"

Personally, I wouldn't actively teach this more formal form to students (at least not below advanced level), but but be ready to admit it's possible if a student came across it in a book, for example. But they should perhaps be aware of it if they are doing CAE or CPE, or equivalent exams.

"Stuff" is neither a plural noun, like "things", nor a collective noun, like "group" or "pack". Stuff, sugar, metal etc are uncountable (or non-count) nouns. As others have said, "many" is used with countable, while "much" is used with uncountable, but for both we usually prefer "a lot of" in positive statements, especially in informal English - "I watch a lot of stuff". But as Benjymyn suggested, it could work equally as well for the negative version. Otherwise I go along with the idea that it's a bit vague, although I wouldn't call it slangy; it all depends on context. But I don't think there's anything 'improper' about it.

@porsche - and rather a good thought, I think. And there might be a good reason. In your second example, we know that somebody (or some people) 'actively' replaced them, so it would be strange to think of the bags as agents. In the first example, however, it's more a process, where perhaps the agent is not so clear. Just another thought.

@Thomas Smith - I have some sympathy for your argument, and I've seen it elsewhere, but in these dictionary examples can we really call the machine or the plastic bags agents? They are hardly the ones doing the replacing. Aren't they just like your robots?

"the manual worker is being replaced by the machine" - the Free Dictionary
"Paper bags have been largely replaced by plastic bags." - Merriam-Webster

Or have these dictionaries simply got it wrong?

@Skeeter Lewis - Here's a thought: use "I've got" etc when you would use other contractions - "I'm", "he's", "they'd" etc, but use "I have" etc when you would normally use uncontracted forms.

@Hairy Scot - I think this may be an example of what you were talking about - I would just call these signs - http://www.dailywritingtips.com/7-vehicular-violations-of-proper-english/

@Thomas Smith - I teach foreign students and have never come across "Enjoy English", but I can assure you that all the major British course books still teach both forms. And I have never, ever seen students taught that "have got" is the Present perfect of "get", because it has very little to do with "get". It's an idiomatic alternative to "have" for possession. That's why you'll find it listed in learner's dictionaries under "have" rather than under "get".

http://oald8.oxfordlearnersdictionaries.com/dictionary/have

In fact if your Present perfect theory is correct, how do you explain "have got to" - the Present perfect of "get to"? It just doesn't work.

In fact it's my impression that we (in BrE at least) very rarely use the standard verb "get" in the Present perfect, without adding something - "I've just got myself a new car" suggests that you have indeed "obtained, bought, stolen" one, whereas "I've got a new car" simply tell us that you have one.

You're right that we can't use this construction in the past or future (which rather proves it's nothing to do with perfect aspect), but you rather confuse the issue by bringing in "obtain, buy, steal" etc, which are all connected with the verb "get", which is pretty irrelevant. "Have got" is simply an idiomatic version of "have" for possession, no more, no less. So the future simple is "will have", the past simple is "had", period.

@Skeeter Lewis - What is a plain man to think? Probably what most of us do (in Britain, at any rate), which is to use "have got" in conversation and informal correspondence, and "have" in more formal circumstances. (see my link to MWDEU)

Mark Liberman, of the University of Pennsylvania, has a post on this phenomenon at Language Log - http://languagelog.ldc.upenn.edu/nll/?p=4274

Strangely, dictionaries don't seem to be a lot of help here. According to Oxford Advanced Learner's Dictionary it looks as though they're interchangeable -
'replace somebody/something with/by somebody/something -
"It is not a good idea to miss meals and replace them with snacks." '

But Longman's has this example - "They replaced the permanent staff with part-timers".- where I don't think "by" would work.

And in this example from the Free Dictionary - "the manual worker is being replaced by the machine" - I'm not sure "with" would really work.

And here are two from Merriam-Webster's:
"I replaced the old rug with a new one."
"Paper bags have been largely replaced by plastic bags."

In fact all the active examples with a preposition I can find in dictionaries have "with", and all the passive ones have "by". So It looks to me as if "with" goes better with active clauses and "by" with passive clauses.

Ngram certainly suggests that in the passive "replaced by" is much more common than "replaced with" - http://books.google.com/ngrams/graph?content=replaced+with%2Creplaced+by&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=

And it seems to me to do the opposite in the active, but I'm not 100% sure about how to do a wild card here. - http://books.google.com/ngrams/graph?content=replace%3D%3Ewith%2Creplace%3D%3Eby&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=

There is a suggestion on another forum that in the passive version, if the thing doing the replacing is the agent, it should be "by", but in the plastic bags example, the bags are hardly the agent, so I don't really go along with that.

And the commenter then gives this example - "Employees were replaced with robots" - which seems to work, but compare it with the Free Dictionary's very similar - "the manual worker is being replaced by the machine"

The Free Dictionary example and the second M-W examples are about general situations, and the "employees" example is about a specific occasion, so perhaps that might also have something to do with it.