I don’t think spelling is a major issue in learning any language, pertaining to what makes it difficult. There are far bigger fish to fry. But, even native speakers struggle with spelling because it has to be taught. Just like reading and writing. But, I agree the best way to improve spelling is by reading. I went Into law school being a terrible speller of English, but came out an excellent speller because of all the massive amounts of reading we had to do.

Hi Aileen ! ... 

I think for me it's not "what is the most difficult feature of English language to learn?"  it's "why i want to learn a specific feature or part of English language ?"  ... because often when someone want to learn English he will not focus on all part of learning language  "listening, reading, writing, speaking, grammar, pronouns" he will just choose some of them and continue with them, because maybe He likes more than the rest of the parts or he need one of them for his work!  ... for example I like listening more than the rest of them . But I think the most important thing in process of learning language is Continuous practice .

I liked your subject so thank you .

In terms of spelling, I would recommend you learn a couple of rules in English wording and vocabulary so that you do not have to learn all spellings for every word even if there may be 10-20% exception.   Remember, you must ensure all words you are learning are all well connected with your pronunciation rather than arrangement of letters, then you may easily commit these words to memory with long term memory.  

Kathleen B. provides good examples to learn spelling effectively and these are rules that have to be well practiced on a usual basis whenever you are reading and writing English. 

Good luck to you

RULE #5 Plurals and Ss

PART 1: No apostrophes! (This is a common error!)

PART 2: Normally plural nouns just add -s (dogs, clouds, rides, conveys, etc.)

PART 3: Special plurals - If words end in -x, -sh, -ch, -z, or -s you must add -es, not -s (boxes, brushes, crutches, buzzes, kisses)

PART 4: For consonant+y words, change the y to i and add +es (baby>>babies; magnify>>magnifies; dry>>dries; country>>countries)

PART 5: F changes to V in plurals (elf>>elves; leaf>>leaves; grief>>grieves; belief>>believes; half>>halves; thief>>thieves;)

PART 6: Verb endings - When conjugating the present tense, the 3rd personal singular (he/she/it) ends in -s  (he sits, she runs)

Those are the main rules for English spelling that will take you a long, long way!