What are some decent alt-tab replacements for Gnome?

Alt Tab is controlled by your Window Manager. Gnome uses either metacity or compiz if effects are turned on. They each behave slightly differently with Alt-Tab. I don't have any delay with either WM, Alt-Tab is quite fast on my system.

If you're using Compiz, you try turning down effects in ccsm or try switching to Metacity. Alt Shift Tab also works with both WMs. I'm not completely thrilled with Metacity's behavior. When going backwards, it initially skips minimized Windows. Luckily, I rarely find a need to minimize, but it's annoying when I do.

Based on your third point, it sounds like the Alt Tab behavior of Compiz which is just like how Windows 7 ruins Alt Tab if I hold on one Window for too long. That behavior can be configured with ccsm under Window Management -> Static Application Switcher -> Appearance -> Selected Window Highlight. Try turning Opacity to 100% and/or changing Highlight Mode.

Your second issue might be a problem with Key Bindings. Key Binding for Compiz are under Window Management -> Static Application Switcher -> Bindings.