In recent years the proliferation of jargon has become altogether too general. Despite its sophistication, formal language must remain clear. Informal English is the language spoken by most people every day. While educated speakers retain their knowledge of formal rules, they're more relaxed about grammar and less concerned with vocabulary when they're engaged in ordinary conversation. Informal writing reflects this relaxation.
Sentences are shorter, and tend to avoid the more formal punctuation of the semi-colon and colon.
Contractions and the first person are acceptable. Newspaper articles and columns are usually written informally, and you use informal language when you write to your friends. The essays of Mark Twain,the novels of Margaret Atwood and the film reviews of Pauline Kael are acclaimed works whose style is largely informal. Popular English can be colourful and highly expressive, but it is out of place in any writing unless you are reporting speech. In everyday speech, imprecision of meaning or poverty of vocabulary can be compensated for by enthusiastic verbal expression, but you can't wave your hands around, or vary your voice for emphasis when you are writing.