Yeah, an afk killer is awesome, just make sure it isn't too early. Give it 30 seconds to a minute for starting and then don't make it go too fast.
Some thing I think:
-3 minutes is pretty good map length. This should equate to about 20 traps and gives plenty of time to do an ending.
-Do original endings. One really cool map that was beta tested at one point had a surf ending and everyone loved it because no one has ever done it before. Try to think of new shit to do.
-Don't add music in. Please for the love of god, don't add music in. It might seem cool the first or second time, but eventually you have to bind a key to stopsound (or whatever the command is)
-Don't just make boring crusher traps. You get tired of them after you already went through 10 of them earlier in the map. A few is fine, but too many gets repetitive quickly. This can also be applied to all types of traps. Try to get a good variety.
-Autotraps. Don't make shitty autotraps that just require memorizing where the floor randomly disappears beneath you. That just pisses people off and is no fun. A small spinner section or miniature parkour part would go a long way towards making your map memorable.
-Someone mentioned it earlier, but don't make all the traps instakill, but don't make them all do 5 damage. You want all of the traps to at least do a good chunk of damage, but too many instantkill traps means that the entire server will do relatively quickly.
-Make it so the traps can be faked. Nobody wants to play the cannonfodder game where people just have to sacrifice themselves to beat the map. A few tougher traps is fun and builds a reputation for that part being particularly hard.
-Try to make a scenery. Maps are a lot more fun if you can recognize where you are. I think most people prefer to get melted by a sudden radioactive waste stream and just get crushed by a sudden dev wall. On the same note though, don't let the scenery make the traps really hard or impossible without knowing just where to jump. If you want to know what I mean by this, look at deathrun_imperialtomb. The floor that suddenly falls is really hard to see where to go after it breaks. The lava part also has a really low grate that you have to jump when you get stuck on because the map maker fucked up and made it too low. Don't let this happen to you because most people think that the map is broken and they don'y enjoy playing on it.
I think that's about it for the advice I can give. I hope I can at least help a little bit.
"There are times when one would like to hang the whole human race and finish the farce" -Mark Twain
“Religion was invented when the first con man met the first fool.”-Mark Twain