What are common mistakes in Tajweed that non-Arabs make?

The most common mistakes are in the letters themselves, meaning using the wrong articulation point for a letter, as well as improper timing of vowels (i.e. lengthening a vowel longer than one vowel count), and natural lengthenings (making them shorter than they should be).  Each vowel over a letter receives one count.  So a dhammah receives the same timing as a kasrah, as does a fath, as long as these letters are not followed by the medd letters (alif ; or a ya' with no vowel preceded by a kasrah,                                    

or wow with no vowel preceded by a dhammah).                     

Medd letters have two vowel counts if they are not followed by a hamzah or a sukoon. 

In this word, each vowel is equal in timing to the next; in other words, this word would have three vowel counts to it.  We can notice that there are no medd letters in this word, so there is no elongation of the vowels.

 This word also has each vowel equal in timing to the next, or three vowel counts. Again, there are no medd letters here, so there is no elongation of the vowels.  In summary, the timing for the first word above is equal to the second word, even though there are different vowels used.   

All vowels, as stated above, have equal timing in length.  This is not true for letters without vowels, or saakin letters.  The timings of saakin letters vary depending on their characteristics.  The difference is in not a great length of time, though.  For the time being we will not be discussing medd letters, but will return to them later. Saakin letters are divided into three groups when determining their timing.  The three groups are:

1.   The group of strength

2.   The middle group

3.   The soft group

The group of strength letters is the group of letters that have imprisonment of the sound when pronounced.  When these letters have no vowel on them (a sukoon) their timing is short.  The letters of this group are:   , and are combined in the phrase:  

 
The middle group of letters is those that do not have a complete running of the sound, yet they do not have complete imprisonment of the sound like the group of strength letters.  These letters therefore have a middle length of time when there is no vowel (a sukoon) on them. The letters of this group are: , and are combined in the phrase:

 
The soft group of letters is the remaining Arabic letters.  This group has a running of the sound when they are without a vowel (saakin), and have the longest timing on them.   

In summary, all vowels have equal length in time, unless a medd letter follows them.  When letters (outside of the medd letters) have a sukoon, meaning they have no vowel, their timings are different in length, depending on which of the three groups they are in.  The shortest timing is in the group of strength , the next longest is the middle group , and the longest is the group of softness .  All letters of the middle group are equal in timing to each other; all letters of the soft group are equal to each other in timing.  All of the strength group of letters are equal in timing to each other, but the two letters  have air released after they are pronounced.