When a stock is "really expensive" VS just "expensive"

When a stock is selling at a PE ratio of 30, nobody can argue: it's expensive. 

When is it too expensive VS just... expensive?

Here's what I think:

There's 3 types of expensive stocks:

1- The stocks that haven't proved nothing yet but for which there's a lot of excitation. Most of these companies never made any profit, or a very small profit. There's no garantee that the stock will make money in the future. We're talking about marijuana stocks, some techno stocks (stuff like Snapchat and Twitter), anything with a crazy hype.

2- The pharma stocks. I really liked these stocks in the past. That was my favorite sector. Now, not so much. I've came to understand that everything is very unstable with these stocks. They're very sensible to politics, to some scandals (a drug sold for too much, an expired patent, a new promising patent finally going nowhere…). So, there's some speculation with these stocks. Some of these stocks may be selling for 30 times current earnings today but they may easily drop to 15 times earnings overnight because of some political announcments.

3- The traditional business in expansion. We're talking here about Buffalo Wild Wings, Texas Roadhouse, Five Below, Dollarama, Ulta Beauty, MTY Food Group, Carmax, and many others. That category of stock does something very simple: it replicates a model that works, over and over. If 100 stores produce 100 million dollars, we could predict that 200 stores would produce 200 million dollars. There's nothing else (almost) to consider. Usually, these stocks are always expensive but their PE ratio stays at a steady level. These expensive stocks are less expensive than the 2 precedent categories because they usually retain their value in the long run.

If I'd understood that 5 years ago, I'd be much richer now.

Better late than never. 

...

Now that I think about it, there's probably many other categories. Otherwise, where could I put Visa and Mastercard? 

But, anyway, the three categories above are frequently considered by investors.