Everyone should carry a multitool. Based on your day-to-day needs, they can get you through all kinds of mini life emergencies. A hangnail, a splinter/skelf, opening a tough packet, tightening a screw on your office desk, etc. etc. etc. Like a good boy scout I've owned a few multitools over the years: a trusty Swiss Army Knife Fieldmaster started me off as a kids, a Leatherman Wave from my time at Bedlam Theatre, a Gerber Suspension that lives in the car, a Gerber Shard that chills out in my rucksack, and a Leatherman Style CS that now lives on my keyring.
Over the past few years, I've collected links and thoughts on features from multitools with the thought that I could someday design my own. I won't be doing this anytime soon - lacking the time to develop the 3D printing/machining skills - but it seems that I've got a fair list of things that I've noted so I may as well pop them up here... here are some of the things I've noted!
So the first thing is that you have to choose how you pack it all together - you go niche with a limited toolset, you go modular and add the bits you're most likely to need, you go separates and hang a few different tools off a keychain or you accept that you can't carry everything you might need. I love multitools where a decent selection of tools for a task are pulled together into one single carryable unit, something that a barman, a sniper or a farrier may need. The addition of a slide clip, karabiner clip or a hole to hang it from some paracord or a keychain are, again, essential. The Leatherman Rime is one of my favourite tools due to its carryability - I don't own one, don't snowboard and don't need one but the combination of boarder-specific tools is amazing: a carabiner big enough to use when wearing gloves, a lace assist (to help you tighten your laces when you're wearing gloves), a board wax scraping edge, a flathead screwdriver for use on board fixings, a bottle opener, a space for clipping on an iPod shuffle and headphone winding storage... an awesome combo for boarders. (The Leatherman Hail is also awesome - it features a space to clip your Leatherman Style into, making a combo tool that covers a whole host of needs!)
Once you've decided on how you'll carry, you can decide what form it will tale. I'm a huge fan of the open-source spork project but having owned and used a spoon-based design it's worth noting that making spoons small is impractical for camping pots, making tools spoon sizes stretches the impracticality at the other end of the scale! If you've gone with large format then wrapping paracord around the handle adds comfort, grip and a new featureset. Size and thickness only matter when the tool isn't the right size and thickness for the job - it's hard to be a spork and a screwdriver for mending glasses at the same time! Keys are suited to small tasks, axes or rhinos to others (maybe!). Same for material and finish - matt black stainless steel might be perfect for one tool, glow in the dark plastic might be perfect for another.
Long blades are a likely arrest issue unless you're carrying a tool for a very clear reason (and rightly so), but short blades can be very practical indeed. Similarly, a sharp corner can let you open most packaging (but destroy your pockets) and a recessed sharp area could let you cut a seatbelt or zipties. I prefer multitools to have some kind of short fold-out blade but if you aim to take it on a plane then this is a total no-go.
Hex bit add-ons or integrated screwdrivers, that is the question! If it's integrated then do not go with just flathead, flathead and crosshead all the way. Lanyard eyeholes can also double up to hold hex bits but having a pocket full of screwdriver bits can be a pain.