I know what you mean. I was at a 2/10 table last night and 3 call the $2 to me on the button with A 10. I want to narrow the field some so I raise to $12. All 3 call !?! I had no idea what any of them had that was worth only $2 a minute ago but now was worth $12. I Folded post flop when I got nothing and there was a raise and reraise before me. All 3 had pocket pairs from 10s down to 5s and could not let any of them go so at least I saw what they all had.
Answers to your questions based on my observations:
#1 Small to mid range pocket pairs is one that I will see alot of people limp in with and then call a raise with. Say you have 7s and you limp in and some raises after you to everyone else folded back to you. 7s heads up has a better chance to win than in a multiplayer pot since the chance of them holding up has increased. Me personally, I would either reraise and assume 2 overs (50/50 to win) or muck (my preferred move depending on situation), but I see alot of people just call.
#2 Suited connectors from 78 and above can have pot odds
if enough people call your raise. If they limp, someone raises and 2-3 call the raise then someone still has pot odds to at least see the flop and run it they don't like it.
#3 People in early position may just call to see what the rest of the table is going to do and hope to get by with nothing more invested or are afraid of a large reraise. In their mind they set what bet they are willing to call if it comes to that.
#4 Idoits, morons and newbees(have to put them there, hard to tell them apart from the other 2). They are the ones who just have not learned to play yet and don't know any better.