Quite basically I will go with mrbelfry's point 4. It's what separates the pros from the ameteurs.
The L in HTML stands for LANGUAGE. Language has syntax and grammar. So does HTML.
I see validation errors (the small pesky, hard to reach ones, ie, unencoded ampersands, etc...) a lot like I see spelling, syntactic, and grammatical errors (of which I make hundreds per day when writing). Not a lot of people notice, but it counts, and it makes you look less educated, and might cost you points in certain situations (professional articles, resumes, etc...). But they don't usually kill you in normal every conversation (like writing a post in a forum...)
However, if you are improperly nesting tags, using non-semantic markup, etc., I would argue that you are not writing HTML. You are writing tag soup.
If you are doing this, I would label you as not knowing HTML, one of the foundations of being a Web designer.
Shifting directions slightly. Why is it that there is no syntax checking that goes on with HTML? When you write in nearly any other language (C, Java, etc, etc...) you get ERRORS if you use improper syntax. This does not happen with HTML. This makes validation errors seem like less of a big deal, but they are the same. They can cause unpredictable behavior accross platforms, or just plain break.
That's my 2 cents. Rife with grammatical, syntactic and spelling errors.