How can I recognize games I should avoid buying if I only play with non-hardcore gamers?
Instead of going on to list one game after another I'll try to find some common points to help you out. I'd say there are three main factors in what might put off less hard-core gamers:


Upfront intimidation (should resolve during first game)
Continuous complexity (remains an issue indefinitely)
"Obscure effects" (should resolve after one or two games)


"Puerto Rico" is a good example of "Upfront intimidation". If you take the time to read the rulebook upfront that's gonna be a huge time investment. For many casual players that might be intimidating having to remember all of it. Having "Lots of bits and pieces" also falls into this category. For some that's a turn on for other's not so much.

"Agricola" is much faster to explain upfront but much more complex during gameplay. Handling all those bits and pieces, making sound decisions and trying to form some semblance of a strategy challenged me big time even after several solo games. There are so many different aspects to keep track of, that it requires a lot of concentration.

"El Grande" would be a good example for what I classified as "obscure effects" (to your choices). The game does not have much of upfront reading nor too many continuous choices. The problem is that it usually takes a few turns before your choices have their effect on your score. Some players don't really mind playing along and waiting to see what they get but others will find it frustrating. Especially in the beginning. This is something you can only find out through reviews or first hand experience.

Boiling it down to a few signs to watch out for: Avoid games that...


have lots of material. (The more abstract the material the worse it is)
take close to or more than 90 minutes. (Anything above that is either complex or boring)