For me, it's punctation. Sometimes I can not work out the exact meaning of a sentence because there are no commas or full stops involved, thereby changing the entire meaning.
As for spelling, well just settle for the make it up approach, it usually works and when that fails, there is always google to fall back on to.
For those who do not have English as their first language, well it is usually obvious, though not always and it can be quite amusing at times explaining idioms to non-native English language speakers...
Scene. In Germany, standing over my touring bike, holding my husband's bike when 2 German cyclists stop to see if we are OK. I tried, in German (a language I used to be fluent in), explaining to them that my husband was simply "watering a tree". 5 minutes later and all 3 of us are in hysterics when my husband creeps back from behind the huge tree he was watering - they have suddenly figured it out!