One of the things that has bothered me is the way YouTube has changed how video's play. Video's no longer buffer the full length, and if you move the slide back to watch a segment over again, it has to reload the video. This has been especially annoying at work where our internet is fairly unreliable and slow. The way YouTube functions now is due to its use of DASH over HTTP.
You can read what DASH is here
If you would like to switch back to making youtube work like before, here are some links to How-Tos for the different browsers. I did it on my work computer with good results. I do have issues, but all are due to either my browser (Opera v12) or our 'interesting' firewall.
Userscript Main Page for YouTube Center
Github Main Page for YouTube Center
These pages contain links to the different files for each browser. Locate the right browser and version you are using. Go to the extension/add-on page when listed.
Note for Firefox users (v19 -- current)
The latest version (V2.1.0) is reported to be broken. If you are having problems, Use the previous version of (2.0.1)
If the howto does not pop up after your first install, here is a really quick guide.
To the top right of the page, you should see a symbol that looks like a gear next to the sign-in button. Clicking that takes you to the settings. To disable dash, go under the 'Player' menu, and uncheck the 'Dash Playback' item. This should be under the 'General' submenu.
Explore the different options and change whatever else you may. You can set the default video resolution, player size, volume level, and much more. Videos have been loading significantly faster for me since I made this change. I also set the 'Player Type' option to Flash, as that worked the best for my setup.