For example Arcane Focus, in the PDF it is described as,
An arcane focus is a special item—an orb, a crystal, a rod, a specially constructed staff, a wand-like length of wood, or some similar item—designed to channel the power of arcane spells.
Note the a specially constructed staff, a wand-like length of wood, and especially or similar item. A player asked me whether his elven wizard could use his bow as a arcane focus. My ruling was that yes it could provided you paid the cost as outlined in the price list. And with the understanding it doesn't allow you to use it as a weapon when you are casting a spell. In other word you couldn't finagle it so you can cast and attack without another rule or ability coming into play.
I posted this on Stack Exchange. And got two answers. The more popular answer was that RAW doesn't allow it. The other answer was to embed a crystal that is a Arcane Focus into the bow.
What I found interesting that the popular answer assumed that if it was not permitted it is forbidden. I looking through various forum posts and google+ posts I seem many answers to rules questions are like that. And then reread the Basic PDF. And that not how it come across. The tenor of the rules is to present options at every point. To make the game your own.
For me personally I viewed the list of Arcane Focuses in the price list as a sample implementation of the rule on page 48. It is there not to limit what Arcane Focuses are but rather to give you something that ready to play out of the box.
Not to say there are not hard and fast rules in the Basic PDF. For example the fact you roll a 1d20 for your initiative is pretty straightforward. But in the backgrounds, various aspects of the characters classes, and other areas of the rules, the vibe is one of making it your own.
In the end I think Mearls and his team did a good job in balancing hard and fast rules with the stuff that can be interpenetrated in a variety of ways.
The key takeaway is that if you are starting out with DnD 5e, look at the rules as guidelines for your own campaign. Don't get trapped in thinking that DnD 5e is meant to be run RAW in the way that 3e and 4e. Even if you don't want to house rule much there is a lot of flexibility in how you implement the rules for your campaign. Particularly in backgrounds, and equipment.