There is only one "tag" that behaves differently on PM vs elsewhere, and that is <code>. <readmore> is an additional pseudo-tag, but has no meaning in normal HTML. (In fact, we make use of the fact that it is meaningless in normal HTML.) I have no idea what behavior you're seeing with <a>; if you give me further information, I can attempt to explain. Perhaps you'll trying to put it in a place where all HTML is escaped.
As to <code>[</code> being difficult to type, you're correct, it is. However, it's rare to mention the [ character all by it's lonesome. When you do, [ is not difficult to type, or to remember. Code tags are useful semantic information, and allow for better visual cues. Please, don't abuse them for formatting.
Allowing input-as-POD, or another semi-plaintext format is easy to do wrong and difficult to do right. So far, we've done pretty well, I think, at not doing things wrong.
Having a tag like code that says "things inside this tag are PODish" is an interesting idea, and I may get around to taking a look into it at some point, but many, many, many things are higher up on my todo list.
Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).