First of all, apologies for the long post!
There are SO many:
1.) Person: 'I've got something to tell you.'
'What?'
(Something happens like a phone call, other person answers phone, guilty look on other person's face)
'So, what was it you wanted to tell me?'
(Long pause) '... Nothing.'
2.) Everyone in the Street/Square/village/whatever must have an affair with each other at some point.
3.) Something will ALWAYS go wrong at a wedding or a funeral.
4.) My most hated one that I posted on a soap cliche thread forever ago:
When two characters who've had like one line of interaction in their entire time on the soap are all of a sudden set to have an affair, said affair shall blossom over the course of around two to three weeks with longing glances at each other that have never once occurred before ever, and the first kiss will either take place and then both parties shall feel guilty (but then kiss each other again the very next day and have sex), or they'll kiss and head straight to bed. Then we may have a period where the bit on the side feels guilty about hurting the significant other, but just can't resist being a bit on the side. And to top it all off... the bit on the side will announce they are in love with the married/taken one after about a week. They'll continue to trade sneaky glances at one another in public places in which absolutely every other person in the public place is oblivious to, even the people standing right next to the couple having the affair, and said affair will conclude with a dramatic reveal, which will usually take place in the local pub which, somewhere along the line, will result in the music being switched off.
5.) The evil grin every time a 'villain' overhears something they can use to their advantage. (Janine in Eastenders was/Tracey in Corrie is notorious for this), or the worried look on the face when a character is hugging someone they're hiding something from.
6.) A whole group of people get together for a "big night out, we'll have a massive party, wahey!" - This night out consists of going down to the local pub and nowhere else.
7.) Any relatives of well-known characters that are coming into the show must never once be mentioned until about a week before the relative turns up.
8.) Almost every pregnancy is a 'Who's the Daddy?' scenario and apparently, it only takes one sexual experience to get pregnant in soap! It seems condoms are non-existent in soap-land.
9.) Everyone always seems to work in the same street. (Corrie's particularly bad at this - does anyone on this forum live in a street where everyone else who lives in that street work in the exact same street?!)
10.) My all-time favourite - every character ever MUST have a 'dark' secret that WILL be revealed at one point.