Another fact that is brought up time and time again, and rightfully so, is WHEN the Authentication server goes offline. No if's about it, just when. You buy the game, you spend half an hour to and hour installing it, go to play, says it needs to authenticate and what do you know - beyond any of your control, the authentication server is unavailable. The server is either down, license expired, or the game is just too old, either way, you can't play.
What about no internet connection? You can activate manually.... through a web browser... *cough*... what? That's like the old joke of "Having problems connecting to the internet? View our online faq's or email us @..." The internet may be ubiquitous, but access certainly isn't. If you don't have internet access to activate, you can go through a web browser instead... logic people... Are you really going to go all the way down to a library or a friends house, just to punch in a ridiculous code, which you may have copied incorrectly, just to get another code to enter in conjunction with your serial number just to be told that you also need to create an online account to activate additional features? Then when you actually get to play, it's so buggy that you need to wait for a patch to fix it... but wait, no internet connection, so how do you know there's a patch and where can you get it? Back down the library/friends place again...
DVD/CD copy protection is bearable, but you'll be left with a bitter taste should the disc get damaged - or god forbid, simply scratched. Best system i know, was used with galactic civilizations series, Just a plain serial number, and when patches are released, you re-enter the serial again. The patch contains a blacklist, so every time they release a patch, new blacklist. No valid serial, no patch installation. It won't stop you playing, but you won't get any new content or fixes. So sure, it gets pirated, and sure, they can hack the patches, but at least the company doesn't pay a monthly subscription plus development cost to implement a rather dodgy DRM scheme, you at least still get to play if there are difficulties with the patches.
Systems like Steam can be useful as well, you buy it once and can play the game on any computer with an internet connection. Emphasis on the latter. No internet, no play.
I guess many would argue that the best system is one that isn't needed...
