Orson Scott Card has discussed, in several places, how prologues (particularly to fantasy epics) tend to be dull, disembodied history lessons. For example, from an interview:
The most common mistakes come in picking where and how to begin their story. Too many people believe that old canard about plunging into the middle of the action: in medias res, the way the classic epic poems began. We see it now in the meaningless car chase, where you don’t know who is in either car, so you don’t care whether the fleer is caught or gets away. The lonely sad person crying, as we then flash back through the entire story.
But the flip side of the in medias res opening is the hideous dump-the-trunk prologue opening, where the writer thinks we won’t understand anything unless we are first told these eight paragraphs (or eighteen pages) of really boring, unintelligible, and unmemorable facts.
Beginnings are all about expository flow — getting information from your head into the reader’s head as painlessly and memorably as possible.
If you have selected the right point-of-view character, then you can simply tell us, as it comes up, what the character already knows and is already thinking about, and then bring up each new piece of information as he or she learns it. If you need a prologue, or a flashback within the first chapter, or a long explanation to catch the reader up, you’ve started in the wrong place.
And most — no, nearly all — first novels make one or the other mistake in how the opening of the story or novel is handled.
To generalize a bit further, I'd say this: Even if your prologue is a great snippet of riveting fiction, it probably feels disconnected from your first few actual chapters. (If it weren't, it wouldn't be a prologue. It'd be Chapter 1.) But the beginning of your novel is a really miserable place to have a disconnected piece of fiction. The reader wants to find out what the story is and what it's about; starting out in one direction and than shifting immediately into another becomes an obstacle to doing that.
As all writing advice, there's no rule without exceptions. I don't recall anybody objecting to the prologue in the first Song of Ice and Fire book, which sets up a major menace and also introduces important parts of the setting before the main plot can get there. The Hitchhiker's Guide to the Galaxy has a short little prologue which is pitch-perfect in setting the tone for the entire book. Understand the risks, know what problems to watch out for, and then you'll have a decent idea of whether your own prologue is problematic or not.