By linguistic features i think of the structure of a sentence, the usage of subclauses, passive or active and such alike.
What would you suggest someone who has no clue of what are considered conventions that ease the understanding of the reader?
A good primer on stylistic conventions is Strunk and White's Elements of Style, at least for writing in US English. It follows a prescriptive convention, which may be helpful to beginning writers. It can apply to many different types of writing, including essays, stories and letters. At its base, it helps to develop a clear and concise style, which I believe is what you mean by readability.
Most writers, however, will advise you to study the conventions of literary style by consuming more contemporary literature. In short, it is not possible to be a good writer without also being a reader.
There are some criteria for assessing the readability of texts, which give you a result corresponding to the school grade at which a child could easily comprehend the text - I.e. the most 'readable' text is the one which can be understood by the youngest children (limited vocabulary, simple structure).
If you want your text to be maximally readable then go for:
- restricted vocabulary
- simple grammar
- short sentences
For simplified English word lists etc, see http://en.m.wikipedia.org/wiki/Basic_English
For interest, here's the readability results for this page as of now (excluding text below this point):
A grade level (based on the USA education system) is equivalent to the number of years of education a person has had. Scores over 22 should generally be taken to mean graduate level text.
Readability Formula Grade
Flesch-Kincaid Grade Level 5.8
Gunning-Fog Score 7.5
Coleman-Liau Index 12.5
SMOG Index 5.7
Automated Readability Index 4.1
Average Grade Level 7.1
This page has an average grade level of about 7.
It should be easily understood by 12 to 13 year olds.
Flesch Kincaid Reading Ease 65
Flesch Kincaid Grade Level 5.7
Gunning Fog Score 7.2
SMOG Index 5.7
Coleman Liau Index 12.2
Automated Readability Index 3.9
Flesch Kincaid Reading Ease
Based on a 0-100 scale. A high score means the text is easier to read. Low scores suggest the text is complicated to understand.
206.835 - 1.015 x (words/sentences) - 84.6 x (syllables/words)
A value between 60 and 80 should be easy for a 12 to 15 year old to understand.
Grade Level indicators
These equate the readability of the text to the US schools grade level system.
Flesch Kincaid Grade Level
0.39 x (words/sentences) + 11.8 x (syllables/words) - 15.59
Gunning Fog Score
0.4 x ( (words/sentences) + 100 x (complexWords/words) )
1.0430 x sqrt( 30 x complexWords/sentences ) + 3.1291
Coleman Liau Index
5.89 x (characters/words) - 0.3 x (sentences/words) - 15.8
Automated Readability Index (ARI)
4.71 x (characters/words) + 0.5 x (words/sentences) - 21.43
Coleman Liau and ARI rely on counting characters, words and sentence. The other indices consider number of syllables and complex words (polysyllabics - with 3 or more syllables) too. Opinions vary on which type are the most accurate. It is more difficult to automate the counting of syllable as the English language does not comply to strict standards!