The biggest problem I had with eve was lag. Fleet battles have no cap on how large they can get, and once you got over 100 ships on each side the delays would start stacking up. You would often be dead before your client synced up. It is much more tolerable these days once they started using stackless i/o. CCP also allows you to "reserve" a solar system (and possibly the surrounding area) for battles ahead of time, allowing CCP to dedicate more resources to that area. I have not personally seen this used though, so I cannot vouch for its effectiveness.
These days I have more problems with my computer being the cause of the slowdown when so much is happening on screen, but that's not lag and not CCP's problem in my opinion. You can turn down all your settings with a few keystrokes in any case, to alleviate the problem.
So in summary, I feel CCP has done an excellent job in this area recently. Back in the day you had to deal with things like "traffic control" making you wait to move around systems. I can't remember the last time I had to do that. The server pretty much never goes down unscheduled. All in all I have few complaints these days.
Dev Blog: Engineering Changes Have Reduced Lag
CCP Explorer has written an extensive dev blog entry discussing the CCP Core Technology Platform and explaining how the significant engineering updates added in Apocrypha 1.2 and 1.3 have greatly reduced lag. "Before we released StacklessIO and EVE64 last year, Jita would reach a maximum of 800-900 pilots, often with a fair amount of lag," Explorer said. "The Sunday after the release of Apocrypha 1.3, Jita climbed to 1,150 pilots with no lag. ... We estimate that Jita can now handle 1,400-1,500 pilots with little or moderate lag."
Check out the dev blog for in-depth information on how the engineering changes came to fruition. As for the future, Explorer said the team is currently focusing on Apocrypha 1.5, which is scheduled to launch in August, and the winter expansion, but emphasized "we will continue to work on engineering tasks such as these in upcoming releases."