There are two types of bad English. There's English from non-native speakers, which can be hard to read at times, but it's not a red flag. Then there's the other type of bad English. It's really hard to describe, but after you've seen it a few times, it's easy to pick up on it after a page or so. It's... a certain type of incoherence and lack of logical thinking, that superficially resembles logical thinking. The difference manifests in the structure of how they communicate, from low-level grammar all the way up to top-level organization of the paper.

Since I can't really describe what I'm talking about, I'll given the most blatant and obvious example I know of: Time Cube[1]. Even if you ignore the content, and just focus on the sentence structure, it's incoherent, often failing to parse as valid English, with a variety of ambiguous or defined referents, and freely introducing new undefined concepts. If Time Cube is 100, most papers you would see in this category are never higher than 3 or 4. But, even at that level, the lack of clarity at the structural level usually implies a similar lack of coherence at the content level.

[1] http://www.timecube.com/

You can find some writing at a level much higher than 4 but much lower than 100 at http://kamouna.wordpress.com/2010/09/03/p-np-iff-p-np/

That was seriously hurtful to my brain.

There is certain aloofness to most cranks that oozes out of their writing.

The things have not changed much since Gardner's "Fads and Fallacies in the Name of Science"

Some of the responses are obvious trolls and he still doesn't catch them.

I disagree. My point is was not that not being able to write perfect English is a prerequisite for good research. However, consistent bad usage to me is an indicator of either a lack of self knowledge/criticism (unaware of one's own level of knowledge) or of indifference, which are both red flags to me. It's an easy matter to have your paper reviewed by a native speaker or an English major from your university.

You perhaps wouldn't argue that attention to clean, professional attire when going to an interview would lead to nasty stereotypes, and generally fashion sense has no bearing with, say, coding ability (I think they are inversely proportional), yet what would you think if someone showed in shorts and a shirt to an interview?

I realize I'm being pedantic here, but you always need to qualify statements like, "what would you think if someone showed in shorts and a shirt to an interview" with "outside of the bay area."

Shorts, T-Shirt, and flip-flops are fine attire for a technical interview at most places you would want to work here. Might not be as acceptable at HP, IBM, or Oracle (or those sorts of places) - but any <500 employee company looking for a technical employee, particularly a Pre-IPO startup, are interested in your ability to deliver, and shorts are completely acceptable [1].

This has relevance to your meta-point though, which is, "know your field's audience, and realize what values they'll judge you on, and take care and attention to deliver on those values - it will demonstrate you have both awareness and the ability to focus on details important in that field."

[1] The [un]fortunate flip side of this lack-of-attention to what you are wearing, is that it comes with an overwhelming focus on what-you-know and what-you-can-do. You can't hide behind a suit and a congenial manner.