The adjective ones I have heard recently are
I was wondering if each of them has positive, negative or neutral meaning?
What are other similar terms that are used for positive, neutral and negative intention?
I will expound on the examples you provided and then give a few more of my own.
forward: Forwardness is not usually considered a positive attribute, as someone who is [too] forward unduly volunteers their opinion or views in a rather annoying manner.
straightforward: A straightfoward person does and says the right thing, regardless of the consequences. Straightforward fellows are usually considered thoughtful (and simple-minded, perhaps), but not rash and stupid.
forthcoming: The adjective, forthcoming is used to describe persons who willingly provide information. It is not necessarily a positive attribute, and it is sometimes associated with disloyalty.
frank: A frank person calls a spade a spade. If especially harsh, such a person could be described as brutally frank.
Of the four words above, I would say forthcoming does not quite belong in the group, as it more concerned with answers than opinions. Similar adjectives that come to mind are:
candid: A candid person speaks the truth in plain terms. This a neutral-positive attribute.
outspoken: This is usually a positive attribute. An outspoken person may speak out in defense of the voiceless or the oppressed and against evil. Such a person may also be a devil's advocate or a rabble-rouser. In these cases, outspokenness may be regarded as highly undesirable.
blunt: A blunt individual speaks the truth without fear or favor. One usually hears the phrase, to put it bluntly. Someone who is not as blunt may opt to put it mildly, instead. Indeed, bluntly-expressed thoughts are usually unpleasant, but they tend to be spoken from the mouth of a disciplined, upright or oppressed individual.
forthright: Forthrightness is a highly desirable quality. It applies not only to expressed opinions but also to actions and attitudes. A forthright person is honest and straightforward in his or her dealings.
vituperative: While a vituperative person may speak the truth, their ultimate goal is to spite or to scorn. This is definitely a negative attribute. There are many other related v-words, such as venomous, vicious, vindictive, etc, but these have broader meanings that go beyond the mere expression of opinion.
I think you'll find quite an interesting discussion of these and related words in the question “Do you agree with these intermediate gradations between ‘frank’ and ‘evasive’?” and its answers.
I also heard people use upfront, as a positive attribute.
Another word would be "plainspoken", someone who expresses themselves in ordinary, easy-to-understand language rather than gussying up their elocutions with a plethora of hifalutin sesquipedalia.
(I should add that "plainspoken" is also largely synonymous with "blunt"; if a plainspoken person doesn't like something, he'll tend to say "I don't like it" rather than beating around the bush.)
There are also colloquial phrases describing such a person. The person might be called "a straight-shooter" or someone who "tells it like it is". Both these phrases can be used positively or negatively, depending on context.
blatant -
