Serrated or plan edge?
Serrations help to cut fibrous materials like webbing, rope, some cardboard, and so on. To have them or not is a religious debate, with "purists" thinking them sacrilegious, and others thinking them essential. While we prefer plain edges on our pure self-defense knives, we're agnostic otherwise. We personally like our police knives with half-serrated edges which is a politician's answer, to be sure.
Manual opening or automatic?
The advertising people in the industry and any number of armchair warriors would have you believe that automatic knives (switchblades) are the only "real man's" knife (sorry, gals!), and that you just gotta cop one if you want to be a real professional, the envy of your squad, and just an all-round sexy guy. We disagree. While the federal ban on autos is just foolish (they were originally designed and sold as housewives' knives, so that the women of the early 20th century didn't break a nail while opening their kitchen knives) and most likely racist (the law was passed in response to the rise of ethnic gangs), auto knives do have a legitimate place in situations where only one hand is likely to be available to open them, with parachuting the classic example. For most people most of the time, they are just ordinary knives with more moving parts to break that are much more expensive than the same knife without the opening spring. Most folding knives today already open with one hand anyway, so the extra expense, agency approval, and potential legal hassle off-duty is just unnecessary.
Another issue with automatics is that most of them open with a push-button. These buttons have two problems. Counter-intuitively, they can be difficult to activate under stress - you'll just have to take my word for it or try it yourself. Second, they can inadvertently "fire" when in your pocket if you brush up against something in just the right way. This can be painful and bloody, and it's always embarrassing. (Benchmade and Camillus make models that operate with a different - and better - mechanism, and they are they way to go if you just have to have a switchblade.) Stick with a normal - and far less expensive - hole, stud or disc operated opening knife. Which you choose is up to your personal preference.
There are four main types of locking mechanisms on folding knives. When well made, they are all reliable and good choices.
Lock-back mechanisms, the oldest of the bunch, have a spring in the spine of the handle with a tenon at the front that engages a mortise on the back of the opened tang of the blade, locking it in place. This is a very secure lock, and the only thing you have to watch for is that the lock does not inadvertently release when you take a very strong grip on the knife - test any you plan to buy.
Liner locks have one of the metal handle liners split lengthwise so that one of the splits jumps behind the tang of the knife as it reaches the fully opened position. For this lock to be secure, tight machining tolerances are necessary. Test any you plan to buy by whacking the spine of the opened blade on a hard surface like wood to see if the lock fails. (Obviously, do this is a way that your fingers won't get cut if the blade flies shut.) Another thing to check is that the lock doesn't fail when - again - you take a super hard, twisting grip on the opened knife. Sometimes the flesh of your palm can inadvertently release the lock. If the knife has an all-metal handle with no scales, and one side of the handle is split to make this same type of lock, it's called a frame lock.
What we call tang locks use a spring-loaded piston in the handle that engages a notch on the rear of the opened blade tang. These are very secure locks that can't be inadvertently released, and we prefer them although we are happy with all of these types of knife locks if they are well made.
Lock strength is an area of promotional competition. Clearly the lock on a knife has to be strong enough not to fail when the knife is used normally, but all folding knives are weaker side-to-side than they are lock-wise, and it's these lateral motions that will usually break your knife if it breaks.
This is where a lot of the confusion and hype, and the genuine quality of a knife can come from. Manufacturers are constantly searching for a better steel, with the result that while some perform better than others, almost all of the knives from the major manufacturers these days use very good steel - certainly steel that's far superior to anything that grandpa could have dreamed of.