Apart from what Cookie said...
Keeping spam away:
Some spammers are caught before they get their chance to even make their first pos, by a script WDZ wrote, But MsgHelp does get spammed like any other forum. Without active mods and admins you wouldn't recognize the latest posts page within a few weeks. Especially now legit members aren't that active anymore the ratio of spammers is only getting higher. We have over 250 IP ranges/addresses, usernames and email addresses/domains banned from registering which is just to keep some obvious repeat offenders out. That doesn't include any of the almost daily spam accounts which do get to post their crap. Those get reported by members or caught directly be mods/admins and perma-banned once someone of the staff gets to it. Usually a few hours at most.
Active staff is the only way, unless your forum is aimed at a single country and you can get away with IP-blocking most of the world (specifically china, india, russia, etc.). Don't rely on captchas or anything which will inconvenience real users. Spammers simply use botnets to get around IP blocks and they can pay people in poor countries a very low fee to type capthas all day.
MsgHelp is in a bit of a strange situation. Technically we use a very old MyBB version, but it is heavily modded by WDZ and he manually patched holes that were later discovered. But for 99% of the forums: keep up to date with the latest version. Make sure your plugins are trustworthy and also keep those up to date. Use strong passwords, and make sure you can trust anyone you give access. http://www.mybbsecurity.net/
has some nice info if this topic interests you.
Keeping things running smoothly:
On the server-side we did run in to some issues. Most oldtimers will remember the "maximum server load reached" messages. But myBB is put together fairly well, most boards won't run into issues. If you do start to get problems knowing how to debug such things is key and you'll be able to fix it. Running on a dedicated server as opposed to shared hosting helps too. MyBB's support forum has very helpful people too if you run into issues.