Ah, Malabo...
For about 18 months, I did rotations based in Bata (on the mainland) and spent a few days at the beginning and end of each one in Malabo. My info is current as of 2010-early 2011.
Flying - most of the fixed wing traffic is back and forth between Bata and Malabo... when I was there, there was a EG company (Ceiba), a Nigerian company, a SOuth African company and on/off other local airlines, as well as military transport operated mainly by Russians/Ukrainians. Lots of heli traffic to/from the oil rigs. ATC is "ok" and speak OK English, but always double check everything... and do your own traffic checks... It wasn't just once where ATC was quite happy to try to send three aircraft to the same spot at the same time!
Climate - hot and humid. I've never sweat so much in my life.
It apparently rains a little more on the island than it does on the mainland, and there always seem to be some argument as to where the weather is "worse" - i.e. Bata vs Malabo.
Cost of living - All I know is hotels are pricey! You can always find reasonably inexpensive local food everywhere... but at the nicer restaurants and hotels you can easily spend $50 US for dinner including wine, etc. Also, cell phone cards seem to get used up way faster than I've seen in any other African country... i.e. easy to use up a $20 USD card in a 20 or 30 minute phone call to North America. Local calls and texts were a lot cheaper, obviously. (To get a sim card you need to go and stand in line at the cell phone office, with photocopies of your passport, as per government regulations (again, this info is as of 2010 or so but since there have been few governmental changes, I suspect this is the same now.) Everything is a bit more restricted, and the government seems to want to keep tabs on everyone, perhaps due to some of the history...) Hopefully though, if you're going as crew for a company already there, they'll just have a local phone all set for you. Anyhow, things in general seem to be more expensive than many other African countries.
EVERYTHING (with the exception of a few of the bigger hotels which accept some credit cards) is cash-based. Be prepared for long lines at the bank (and everywhere else!) unless you have someone at your company that does that for or with you (can be really overwhelming if you don't.) Euro and USD are easily exchanged.
Culture - Spanish is the main language, but French is the language of "business" and a good number of people speak that as well. The occasional person will speak some English. Equato-Guineans come across as somewhat reserved but are polite. You're rarely "bothered" on the street, and I found I was often left to my own devices unless I engaged someone in a conversation and/or ask for directions, etc. The business-people dress very elegantly for pretty much everything, work, travel, etc; you'll see many suits & ties and dresses & heels. They are very proud of their country.
Don't talk politics... the President is revered... and is very security conscious. THat's all I shall say about that!
The airport security guys in Malabo are used to a lot of foreign workers who speak no French/Spanish/English going through, and have realized that they can easily intimidate them into handing over cash. Just be wary of this, keep your cash and passport on your person as opposed to in a wallet you have to put thru the "scanner", and allow yourself enough time so you can just sit and wait them out (they do get tired of waiting!) There is also a lot of corruption when it comes to shipping/receiving offices and some of that is unavoidable. Just be aware.
Diseases - Malaria is rampant. Again, my info is mostly for Bata but I expect it's the same for Malabo... There's an excellent Israeli/Spanish hospital in Bata, and it's always full of malaria patients. During the year and a half or so that we had a crew there, we heard of several expats dying of Malaria so be careful, and go to the hospital/clinic for a test at the first sign of it. OTher than that, just the usual "don't drink the water" and expect bedbugs...
Security... I never really felt unsafe there but I was always careful. Just usual precautions... and again, the folks I came across were typically polite and/or just left me alone (for perspective, I'm female with light skin.) As far as police go, they have a few checkpoints here and there on the roads and our drivers never really had any issues with them, except for the occasional drunk military policeman waving around his (likely unloaded) rifle asking for a soda. Always carry a copy or two of your passport as well as the actual document itself, but only present the copy to police when asked... other than that - again, just the usual precautions for any big city.
Anyhow, I hope that helps. There are a few good books on the country that would be worth picking up (a couple are banned in EG itself so be careful!) but other than that, I think I've covered most of what I figured out in my time there. It's a really interesting country, with beautiful jungle and a volcano towering over Malabo (when it's not covered in cloud!) and it'll definitely be an experience!