A number of years ago, I took a tour of a DVD-R manufacturing facility in a nearby city.
I spoke with their product engineer about the compatibility problems of the burned discs and here's basically what he told me (paraphrased from memory):

First, burned discs are 2048 bytes per sector and true replicated DVD discs are 2056 bytes per sector. Some set top players cannot understand the 2048-byte sector and will simply refuse to read them.

Second, there is a bit of data written at the replication factory to the start of every disc. This information differs from that which is written to "format" burnable media.

The above two are in addition to the more commonly-known reasons having to due with reflectivity differences and improper encoding.

I feel the need to correct the misconception that DVD video bitrates need to be kept below 7mb/S in order to have a playable disc. We typically create discs with bitrates that reach 10.02mb/S on a regular basis and our clients have no problem playing these discs. Limiting to 7mb/S just limits your picture quality--fast motion sports and complex scenes will suffer block artifacting.
The best way to lower the bitrate is to shoot 24 frames progressive, encode as such and change the flags to 60i with DGPulldown and author.
Another thing we found was that disc compatibility went up when we started authoring in Scenarist a few years back. The consumer DVD authoring apps simply don't adhere 100% to the DVD spec. Nowadays, we're testing Encore for authoring Blu-ray discs on the cheap and compatibility is very low--only about 55% of the players I tested at Best Buy will play the disc. You'd think the software and hardware manufacturers would have learned from the DVD-R experiences.