I mentioned on IRC the other day that there's a difference between less than perfect grammar, and bad grammar. Less than perfect is something like we'd use in normal dialogue. The grammar rules are usually more of a guideline than a hard and fast rule. It can be bent a little, in the right circumstances. Bad grammar, as mentioned by Lynn and Wendy, has the effect of throwing the reader off completely. Most readers will either be confused, or (as sometimes in my case) start shouting corrections as they read. This type of bad grammar makes the story unreadable.
However, grammar can be bent into knots when it comes to dialogue. After all, those particular instances may be dialect or a reflection of that character's upbringing, intellect, or age. After all, a two-year-old isn't going to be saying perfect sentences with perfect grammar.
As far as the introspection debate goes, if it's done right, then I'm all for it. Granted, I don't want pages of introspection with no action, but if mixed properly, it's a good combination.
