If you're just starting out, to avoid problems, you probably need to be MORE cautious than somebody with a lot of history in the industry, rather than LESS cautious, until you're totally comfortable with what you're doing.
The safe approach is to stick 100.00% to product/service domains that describe something in completely generic terms. By generic, I mean the same way that the dictionary would describe them, and that every company in the industry describes their own branded versions of that product/service. You're looking for terms that do not depend on the efforts of one particular company to make them popular.
A few examples may make this a bit clearer, I hope...
"fitness workouts" is generic. Nobody has created a brand around "fitness workouts" because it just describes the actual "thing" in normal, every-day language. "P90x workouts" (as has been covered in depth on this thread) is not generic, because the interest - and value - of the expression has been created "artificially" by one company.
"operating system" is generic. But an expression that included "microsoft" or "windows" (in a context that could be interpreted as referring to software, not the glass windows) isn't generic because Microsoft has created the value and the interest.
"free email" is generic. But "hotmail" is a brand that has been created through the effort of its founders, and later by Microsoft after they bought the company.
NOTE: the fact that something is incredibly popular and talked about absolutely everywhere is a very, very poor measure of if something is generic or not! You simply MUST go back to basics, think of the dictionary, think of the industry, think of the products/services within that industry and how EVERYONE talks about them (everyone = all companies competing within that industry) and that's where you'll get the real clue as to whether something is generic or not.
A few more examples...
"mp3 player" > generic; "ipod" or "walkman" > brands
"car parts" > generic; "volvo parts" > brand (the expression only means something because Volvo the company has made it mean something)
"diet programme" > generic; "atkins diet" > brand
Another way to think about it is to imagine filling in this sentence...
_______ (brand) is a ______ (generic)
Hoover is a vacuum cleaner (Dyson is a vacuum cleaner etc.)
iPhone is a mobile phone (Nokia is a mobile phone etc.)
EasyJet is an airline (Ryanair is an airline etc.)
Heineken is a beer (Grolsch is a beer etc.)
I've bolded the generic in each of the above examples. As you can see, every company in that niche would describe their BRANDED product in the same GENERIC way. It's that GENERIC expression that's safe to work with.
I hope this is starting to clarify the difference between generics (safe territory, worth investing in if the product/service they refer to is commercial enough, but likely to already be registered) and domains that include someone else's brands and trademarks (dangerous territory, best avoided)
