How bad does it sound if I use
個 for everything if most measure words are just still unknown to me, or I don't want to end up using a wrong one?
How bad does it sound if I use
You should learn those measure word one by one because most of native speakers don't use "個" for every nouns. But sometimes if you forget or don't know the word you should use, most people still know what you're referring to with the word "個" in most case.
Why I said in most case? Here's an example: (Actually, it's a joke.)
The foreigners usually have a big trouble if they should use "個" and "子". "個" is a measure word(量詞) when you have no idea which word you should use while "子" is something we add after a thing for better reading, like "桌子", "椅子".
So, one day. Tom walked into a watch store and ask for a new watch: "老闆，我想要買一個錶子。"(Lao ban, wo xiang yao mai yi ge biao zi).
I kind of understand why the negative votes on zyy's deleted post, because no native Chinese speaker will tell you it's correct to use "個" as a "cure-all" silver bullet if you don't know what the measure word is. It is not CORRECT, but is it definitely not OK all of the time? I have a slightly different take on this.
I believe language learning is a process, a very long process for that matter. Making mistakes, and the willingness to take risks is part of that process. As a language teacher, I will NEVER tell a student using "個" for everything is correct, but neither will I discourage him/her from taking risks. If you are in a situation where you either use a word you're uncertain of, or keep silent, then I'd say assess the situation and make your choice. If this is a very formal discussion, and making a grammatical mistake will make you very uncomfortable, perhaps say nothing until you find out the correct way to say it. But if you take this as an everyday learning experience, and you're among friends, why not take the risk and learn something? Again, it's your willingness to take risks.
As said above, using "個" for everything is grammatically wrong, but most of the time, you can be understood, eventually. It's even a little better than skipping the measure word all together.
A comparable question that comes to mind would be: if you're learning English and don't know the past tense form of a verb, can you just tag "-ed" onto the present form and get away with it? Again, maybe, but whether you want to do that depends on your willingness to risk making a mistake.
Having said that, measure words are not all that difficult because there are some semantic clues that can help you make an intelligent guess as to which measure word to use.
Using 个 for everything is not right. But how bad does it sound? I would say in colloquial setting, and when the fact is clear that the speaker is a language learner, it doesn't sound too bad, or not bad at all. Thanks @monalisa for the wonderful analogy, it's a lot like using "-ed" on every verb. If I say "teached" instead of "taught" as the only ungrammatical part in a long sentence, I can probably get away with it more often than not (i.e. it wouldn't bother the listener as much as I need to interrupt the speech and make up for it).
The reason why this analogy works so well is probably because like "-ed" is called the regular form, 个 is the "regular" measure word. It's the most widely used one, and the default one when a new noun emerges. Not all the wrong usages sound equally bad; 一只人 sounds much more awkward than 一个鸟。
Another angle to consider is, just like irregular forms in English is gradually going away , no one says brethren or holp anymore, 个 is gaining popularity over the less common measure words especially in spoken language. It's actually very common to hear 一个电影, 一个啤酒, 一个飞机, etc. Those must be wrong to start with, but are they still wrong when they become the idioms that everyone is (mis)using? Or is that just how languages evolve?