If your website makes extensive use of Flash, fire your web developers and hire ones who know what they're doing. Web developers who rely on Flash are doing you a disservice: Your site won't work on iOS or many Android phones, which will drive those people away. Tell your restaurant-owner friends to stop paying web developers who rely on Flash and PDFs.
If you must use Flash, degrade gracefully. A recent BBC article on big wave surfing had a video of a monster 72-foot wave. When rendered in a browser that doesn't support Flash, the video player was replaced with an image and a note saying the video would have been shown if the browser supported Flash.
At least the BBC article looked good and welcoming.
Frankly, I wanted to read this story in the graphic at left, but when I see a popover, I leave immediately. You got the ad impression, but that's it. When I leave, there's no chance that I will interact with your site further and there is less chance I will return, ever--at least, not willingly.
"Under construction" notices. Make your content work with mobile devices, period. A page like the one at left may seem helpful, but it's off-putting. Even more so because I clicked through anyway, and the page rendered fine. I bet the trigger was a Flash video widget. (See No. 1.)
Most modern mobile browsers, at least the ones I use on Android, anyway, render "desktop versions" of sites really well. Sure, visitors have to pinch in to see the text, but browser UI elements like magnifying a touched area of a page, smooth zooming and improved character and image rendering make getting to your content easy and reliable for visitors, without telling them you couldn't be bothered to support mobile devices.
The upshot: If you aren't coming up with a mobile-friendly design for your website, you're driving visitors away. A well-designed mobile site lets visitors on small form-factor devices connected via potentially slow and congested networks see your content and interact with your site quickly. Figure out which components are most useful for your visitors and design your site accordingly.
As Brian Katz, who heads up the mobility group for a global pharmaceutical company, says in a recent blog post about application development, "know why you are creating an app, figure out what data you need to access and how you will do that securely and then worry about the best tool to use for building the actual app. Don't spend so much time working it backwards."