What is is that make a good anime/manga for you?
Is it a story, animation, character development, something new and unseen yet, drama, fan service, crazy skills and talents of characters, comedy, badass fight scenes or something else...
I believe its a combination of several factors, the story is a major part of it as without a decent story line the anime is doomed to begin with. The authors ability to develop a character and also make them relate able and accepted by an audience so the audience feels for the character in times of pain and joy. A good twist every now and then is a good thing but it shouldn't be made into a habit like it was over done in Bleach with Aizen. I personally enjoy action adventure anime, it can't just be pure brutal killing but it can't be all rainbows and puppies either a good balance as well as good and steady character development has never failed me. The anime can't reveal spoilers in the opening like in Fairy tail that just spoils it but a good catchy opening can really motivate me to continue watching a long series. Anime also have cliches that i really enjoy like the group gathering after a fight and the designated character roles like the emo chick and comic relief idiot.
I believe it is mostly a combination of those different things, with bigger emphasis on story, characters & comedy (personal choice, but who doesn't like to laugh?). A series that can make you feel a number of different emotions: joy, sadness, excitement, and make you laugh and keep you at the edge of your seat will always make it to the top of my list.