its not about one against another. Having a competitor as big as yahoo or microsoft or google isnt a integrator for deciding if it will be successful or not.

google went against huge competition and won.

i think from my perspective  what makes a web app a successful one is that it solves a problem. Any problem. even if you need to order bread more efficiently using a web app., that will make that website successful .

you need to solve an issue and make the users come back. everything else might be successful also but probably in the long run wont stick and hold a database of users. there is a limit to how much you can hold a user without giving him some value for his time spent on your web site. 2.0 , 3.0 or 77.0. its the fundamentals of building a community.

Guy kawasaki once said a successful app is one that serves as a solution for an everyday life problem. google changed our way of life in work and in the way we look for stuff.  thats what every web app inventor should strive to. serve a user with a value for his time.

ofcourse there are issues with design and simplicity. you need a good design to stick that doesnt demand too much from the user as far as learning curve. you can have the most innovative smart application that cracks the atom. but if its full of buttons and text that you need to learn you'll probably miss 98% of your crowd just because the user will get tired. you dont want the user to feel stupid that he doesnt understand your website because you think its simple.

make sure you do alot of usability testing on your webapp and give diffrent people access to your webapp and listen to their comments. dont think you know it all. 

Alot of features arent the solution. finish off a few good solid features and offer them. dont spread your webapp onto too many features because the client wont understand what your about. take flickr for example. im sure they could offer video uploading. but they dont. because they are focused on their main purpose. stay in focus. 

as far as preferring a website, you always need to offer some kind of twist. what does apple offer more then microsoft? they both give you the ability to surf and play games. apple just does it quicker with bells and pretty colors. you can do the same as yahoo answers but simpler or more efficient. if its the design or adding a few more features. 

For example, Yahoo answers doesnt let you invite people to answer questions. the reputation method there is pretty lame .

You see, its all about offering something new or improving the old one. giving a user value for his time.

pewww that was long.