Some of the mistakes a beginner can make:
1. too occupied with fingerings  -they just forget that they are playing music. 
2. sometimes they just doesn't listen to the tune -too reliant on tabs all the bloody time
3. neglect to practice the basics
4. too eager to play the advanced stuffs
5. playing a single musical style -mix it up; preference of one kind of music shouldn't make you a snob towards other musical styles

Yes. There's few exam boards offer guitar exam, eg: Trinity, ABRSM, Yamaha.

Trinity is as equally recognised worldwide as ABRSM. Yamaha grade system is also widely recognized, but yamaha is unique, its grade system go backwards, grade 10(fundamental) grade 9 to grade 7 (intermediate), grade 6 to grade 3 (advanced). Beside classic repertoires at higher grades, we learn music arrangement and improvisation.

It's compulsory to pass grade 5 theory exam to enter ABRSM grade 6 and above practical exam. Trinity and Yamaha do not require this.

ABRSM exam focus more on the technical skills, lots of scales scales scales and arpeggios. ABRSM and trinity do not have accompaniment test, but Yamaha have this, examiner plays the melody, you play chords and figure out the rhythm.
If you sit for ABRSM or Trinity guitar exam, the examiner may be not a guitarist, he/she may be a pianist or violinist or other instruments player. However, if you sit for Yamaha guitar exam, the examiner must be a guitarist.

1. Yes. That's what music is about -something that you want to listen to. Now, (psychologically) we all *hate* the way we play -why can't I play like such and such, why is that kid plays better etc. But we should listen to our playing to identify our weakness and correct our techniques:
One musician said that his practice always include playing and holding one note on his instrument for minutes (he plays alto sax) just to listen to his playing of that single note -he was going for the tone i.e finding his own voice. Try it on the instrument of your choice
2. Not sure what your question is... Tab here means a simplified music sheet for guitarists, it usually shows where to play the notes, what chords to play etc.
3. Basics, what finalzz posted.
"too occupied by the fingerings"
Remember when we started playing we can't keep our eyes off the fret? Of course looking at our fingers do help improve our playing, but it also leads to improper playing posture and neglecting the picking hand/fingers, these two problems are what I see most in beginners.
"beginners should start somewhere by reading tabs"
That's it! When I's a beginner, I never *read* tabs, I just follow the numbers and this went for months or perhaps years -listen to a piece of song, google the tabs, spend hours of practice memorizing the numbers, by the end of the day I just play a bunch of numbers not the music.

p/s Of course this only applies to self-taught guitarists .
People who attend classes don't know what they're missing;)

well said. a musician in general should be playing music, not blatantly and blindly reading notes. but then again, classical instruments often read notes. now don't get me wrong, i love classical instruments and stuff, but what really matters is the idiosyncracies of the player, channeling the original emotion. IF the player manages to understand and channel the original writer's emotions but reading through tabs, then i personally think it's ok....

posture is VERY VERY important, didn't find out the importance of it until i started learning jazz guitar. slouching is not acceptable, and it's most recommended that players wear their guitar straps while they practise, so they'll get use to the motion of the guitar when standing up.

faceless, tabs as in the numbers which represents the fret and string, not what you think it is