For the most part, they're economically unfeasible. Not only do they lack variety, but they tend to be run down. This past July 3, some of these places had Independence Day playing on as many as 8 screens. With staggered starting times, you could walk in off the street at any moment and be no more than 15 minutes away from the next showing. The quality of these megaplexes varies.
Some seats are more comfortable than others.
Employees can be courteous and knowledgeable or rude and apathetic. Still, even the cleanest, most modern multiplex will have trouble drawing customers if it isn't conveniently located. People are generally reluctant to change their movie-going habits. I tend to be very picky about where I see movies, and I frequent more than a dozen theaters, but I know that I'm in the minority. Most of what I've said so far applies to mainstream movies.