OK there are alot of points that make an anime great. But most are based on ultimate opinion. Such as character fans [mostly girls not sexist] are attracted to shows like Code Geass,Kimi No Todoke,Amagami SS and the list goes on.But plot fans will go to shows like Higurashi and such.
Now shows with amazing plot and characters with a great op and ep such as Clannad,Kanon,Air,Darker Than Black,Angel Beats and so on will typically attract alot of people or a solemn few who really truly know their potential.
Fanservice is an issue with some shows where they go way overboard on the ecchi stuff [you know what i mean] like everyone thinks a little is good but alot is just ruining the entire anime.Lets seee a good overboard ecchi show...... To Love Ru!!! is an ok example i felt that that show was on the limit personally but ya a little service is good.
Harems are good from some of what ive seen,this is where the overboard ecchi comes in where all the girls are somehow getting naked "accidently" and showing everything just to pretend to be embarrased.But there are alot of good harems that have a really great story like Shuffle!.
OPs and EPs need to really grip people or bad things will happen.Having a good OP reels the watcher in and sticks him for the rest of the show,if you have a bad one most people will trash it.EPs are neccesary too because if you dont have a good one then we just feel like a waste of time. I know im stating the obvious but there are some who dont really get this stuff.
Whiny and useless characters get on everybodies nerves.nuff said
My opinion for Genre:
Action-Darker Than Black,Code Geass,Angel Beats,Utawerumono,Black Lagoon,alot more cant think
Drama-Darker Than Black,Clannad,Air,Angel Beats,Kanon,Futakoi Alternative,H20-Footprints in the Sand-alot more
Comedy-Angel Beats,Baka to Test,Mitsudomoe,MM!,Azumange Daioh,Lucky Star,-alot more
Just plain gotta see-Darker Than Black
These are just off the top off my head im too lazy to do the research
Two To The Chest One To The Head