Spiteful. Petty. Mean-spirited. There are some characteristics that have no redeeming features. Patronising. Superior. Condescending. Give your main characters these qualities and you can guarantee you're going to turn readers off. Subsidiary characters can of course have these qualities, then we like the main characters for giving them their comeuppance.
Now, I can hear you all saying, but what about the character we love to hate? The character we love to hate usually has attractive characteristics in spades - qualities like energy, flair, wit, sex appeal. They may do bad things, but that doesn't seem to matter as much.
2. Characters who are nice
Bland characters are boring. Inoffensive = unmemorable. Nice people are good to know in real life but they make heavy reading. Quick - name a woman in a Dickens novel. Bet you came up with Estella, Miss Havisham, Betsy Trotwood, Bella, Sarah Gamp, Nancy, Lady Deadlock - all characters who aren't particularly 'nice' but are memorable. Anyone remember Dora? Nope, didn't think so.
3. Characters who react rather than act
Main characters need to do stuff, not hang around waiting for things to happen. We can all jog along nicely in our own little ruts in real life, but who wants to read about it? Your character might be in a rut at the start of the book, but once they've been thrown out of it (as they surely will, and hopefully pretty early on) they get going and start doing.
4. Bad first impressions
I am sure there are lots of gorgeous men out there called Geoff or Brian. Or Nigel. But it's hard to think of them as leading male characters. For the women, it's hard to get excited reading about Mildred, Doris or Blodwen. Names are important as they have all sorts of associations - Scarlett O'Hara was originally called Pansy, which doesn't have quite the same ring about it.
It's also hard for characters to shake off negative impressions if they're shown displaying unattractive characteristics (see no 1) the first time we see them. You know the saying that we make up our minds about someone within 30 seconds of meeting them? Same is true for reading about them, and it's very hard to shift those first impressions.
There are still some places on the How to Sell a Novel day course in Bath on 3rd July - clickhere or contact firstname.lastname@example.org if you want more info.