I've been accepted to a top 20 Computer Science department with a RA, TA and fellowship. I decided to defer my admissions a year due to unforeseen circumstances. I was informed the school does not allow deferment, and my adviser told me to simply reapply next year. If I was accepted to the university this academic year, should I be able to get accepted next Fall? My application will be the exact same as it was this year including my recommendation letters, personal statement, and research experience.
I'll address the chances of being admitted again next year at a school that accepted you this year. Certainly they aren't 100%, or even 95%, since if they were that high, then the department would save itself the trouble and simply allow deferrals. As Aeismail points out, it also depends on the strength of the applicant pool, and that can fluctuate from year to year. For example, in a bad economy, or if the department hires a star faculty member, it can become harder to get admitted.
Department admissions procedures vary greatly. In some departments, having strong support from a single faculty member may be enough to guarantee admission. That is not true in my department, and I'd estimate that the average chances of being admitted again in this department are about 75%. (Don't take this figure too seriously, but it's a better estimate than 50% or 100%. It's based on experience and thought experiments rather than data.)
Roughly, the top 25% of the class is admitted quickly and easily, with little debate. The next 25% involves more discussion, but the answers are probably pretty consistent, and the main issue is ranking for things like fellowships. It's the bottom half that involves increasing amounts of randomness. The number of plausible acceptances - people where nobody would think it strange if they were admitted - is substantially larger than the number we will actually accept. Near the dividing line, there are a lot of people whose applications are really pretty comparable. We arrange them in order, after a lot of discussion, and draw lines for admission and waiting list. The order is not completely random, of course; the stronger your application is, the higher you are likely to be ranked. But there's a lot of arbitrariness to it, since it's a matter of small distinctions. The trickiest cases are the applicants who have notable strengths and weaknesses, and their positions depend sensitively on who is on the committee: getting a high ranking depends on having some committee member who strongly supports your case, and nobody who is strongly opposed. (Admissions committee are often risk averse, so opposition counts more than support.)
Overall, I'd guess that as many as half of the people in the bottom half of the list might have been replaced with someone else if another committee had examined the applications from scratch. That would correspond to a 75% overlap between the decisions.
Of course, this figure is pretty much meaningless for individuals. If you are a star, then you will definitely be accepted again. If you were the most controversial and lowest-ranked person last year, then your chances of being lucky next year as well are not good. The truth is most likely to be in between.
If you have the time and financial resources to apply, there's no reason not to apply to schools that you think you have a chance to be admitted to. One thing that you could do is to see if you can get an appointment to talk with a graduate admissions officer at one of the departments you're interested in; you'll find out pretty quickly what the officer thinks of your admission chances. (It was how I learned not to worry about my credentials when I applied way back when.) In addition, the initiative may be appreciated by the officer, which may help your chances of admissions in the long run.
As for deferment policies, those are strictly school- and department-dependent. I would hope that you would be considered equally both years, but the truth is that you are usually compared to other applicants in the year that you apply. Therefore, a weaker application next year will not help. (Although if your grades in the major are strong, weaknesses outside the major won't hurt you so much.)
I would assume that your chances to be readmitted are not high: the graduate committees would probably remember that you did not come last year, and will not want to risk another deferral. It is not like a faculty position re-application in which the recruiting committee has more time to think through process, and would have time to evaluate the chances that an applicant that deferred in the past would come this year. In graduate school admissions, I believe decisions are made quite fast.