Lessons learned
1) Shooting at stationary structures is boring
See: Starbase warfare, Dominion sov warfare. Even the good fights that do happen around such objectives could be improved by having better objectives.
Shooting at things with hitpoints scales very efficiently with fleet size, which encourages lag-producing behavior
2) Having to spend significant amounts of effort defeating an enemy which isn't even fighting back is really boring
See: Starbase warfare, Dominion sov warfare. See in particular how long it took to clear IT Alliance's ownership out of Delve, as a recent example
3) Waking up every morning and having to clean up the mess made while you were asleep is boring
See: station ping-pong pre-sov, repairing station services. Having to do something tedious every day before you can actually play the game is not cool
4) Doing something just "because it would be cool/neat/awesome" is always a bad idea and will come back to bite you later
See: Jump bridges, cyno jammers, Sov 4, AoE doomsdays, titans in general, supercarrier boost... Note that we should still obviously strive to make everything cool/neat/awesome, but when we start off with an awesome idea rather than an actual problem we want to fix or a feature that has a clear, functional and necessary goal, it generally requires painful fixes further down the road
5) Cost is a useful variable to tune but an unwise thing to rely on to enforce scarcity or balance - players will always be richer than you think
See: outposts, titans, supercarriers
6) Making something tedious will not stop players doing it if it's very clearly the best option. They'll do it, and they'll hate it
See: everything involving starbases. As a counterpoint though, things like the one-per-corp-per-system-per-day starbase rule demonstrate that if something doesn't make a big difference but is sufficiently awkward to do, then any theoretical "exploit" scenarios tend to fall out of favor quickly as they're just not worth the effort.
7) People like to do one-stop shopping, and will "go to Jita" for everything unless doing so is comparatively very inconvenient
See: moon mineral distribution, high-strength booster resource distribution, neither of which achieved much in the way of the nullsec-to-nullsec trade that they hoped to encourage