There were several issues with ETQW, first - it is heavily "overobjectified" game - it has a lot of objectives and unlike most team shooters it actually needs all classes to be up to scratch with tasks or it will fail miserably. It also offers very little help for first time players, which are basically thrown straight into action with incredible amount of detail and just about anything you learned in original Enemy Territory goes out of the window. I would say it has one of the longest learning curves among team shooters atm. Which means first time players and foreign players who can't understand fast pace instructions appearing in detail on screen get discouraged very quickly. Simply setting various difficulty levels (less and more tasks/targets to achieve per map) on server side would probably help matters.
But the initial server side release was a mess too. You couldn't specify campaigns with all maps or indefinite XP saves, you couldn't customize them, so basically there was no carrot for Enemy Territory players. In fact it would be probably better if it was actually called Tactical Quake Wars, and cut any ties to expected simplicity of ET. Having said that, I recently picked up second £7 copy for my workstation and after running single player with all maps in a loop for few days I have to say, the game is under appreciated. It's a shooter that missed the target audience - it was marketed as team deathmatch, so most people would join servers and find most of the players blasting heck out of each other and campers sniping from towers instead of following any objectives, but it can be easily made into fantastically complicated team game for adults with right set of people.