In my area, many of our OTA network stations share a channel (due to financial issues, not to free up spectrum). Fox/ABC, CBS/CW, NBC/myTV.
I think most programs have no issues with 12+ Mbps, which usually leaves room for a SD sub-channel. However, shoving two HD programs into a 6 MHz channel leaves each ~9 Mbps. Sports programs suffer significant blocking and pixelation on fast action and pans. Live shows such as America's Got Talent also block and pixelate. Studio shows fare better.
I believe the problem may vary based on market since, I assume, the broadcasts are re-compressed locally and we're at the mercy of the capability of whatever system my money-starved stations purchased. NBC and CBS are the worst here, and I'm not sure if it's their choice of compression hardware or just that 1080i suffers more than 720p.
So basically on these shared channels, you suffer from limited bandwidth, re-compression artifacts (going form 12-18 Mbps down to 9), and local hardware limitations (poor quality compressor, no pre-processing/single-pass only, etc).