Tarantino was originally planning on filming the movie with his friend Lawrence Bender and a $30,000 dollar budget in 16mm film (which is a cheap kind of film not normally used in theaters). Then when Harvey Keitel got interested, the budget got a big boost all the way up to $1.5 million. What you might be thinking is, "Dang, that's a lot more money," but what you should be thinking is, "Dang, that's still not much money." Filming movies ain't cheap, so to work within the budget (and the timeline the budget enforced) Tarantino had to cut a few corners.
First up: the heist. One of the most famous things about Reservoir Dogs is that you don't actually see the heist itself; the movie isn't about that. Also, while the final decision was in part an artistic one, initially it was simply a lack of money that lead Tarantino to not include what would have seemed to be the most critical part of the film. Really, then, the small budget drove the entire course of Reservoir Dog's narrative.
There are other things that the budget impacted as well. Take Mr. Blonde's car, for instance. The car was filmed in the scene where they extract Marvin from the trunk outside the funeral home. They were going to have to rent a car, but Michael Madsen (Blonde) said they could use his car to do the shot. Apparently, he still has that car today (source).
Nice Guy Eddie's suit is also a borrowed prop. Chris Penn, who plays Eddie, said he owned the suit and that it actually wasn't too far off from what he used to wear at the time. In fact, the suits they wore were provided free of charge by an American crime film enthusiast. Steve Buscemi, who played Pink, actually wore his own black jeans in the place of slacks (source).
You know that nice blue-walled apartment interior used for some of Orange's scenes? Instead of spending money renting an actual apartment, they simply used some of the upper rooms in the funeral home where they were filming other scenes. They splashed some paint on the walls to spruce it up a little bit and voila, a nice little apartment that you would never know was the top story of a morgue. Yes, the funeral home was actually a morgue… that was dressed up a bit to look even more morgue-ish.