I'm a bit of a mechanical keyboard nerd, I collect keyboards with all sorts of form factors and switches. I know it can be quite challenging to find your first mech, and to make sense of all the terminology, so I thought I'd write down a short guide to help you through your selection process.
First of all, you need to decide on a form factor. Generally speaking mechanical keyboards come in 3 form factors: full size, tenkeyless (or 87%) and 60%.
Full size keyboards of course have all 104 keys as defined in the ANSI keyboard standard. You should always look for a standard key layout without a weird shaped enter key or other weirdly placed or shaped keys.
Tenkeyless keyboards are like full size keyboards but with the numpad removed. This makes them more compact, meaning they take up less deskspace and more importantly, you can place your mouse in a more ergonomical position closer to the alphanumerical section of your keyboard where your hands will be most of the time, so when you grab the mouse, you have to reach out less far. This is by far my favorite form factor, and unless you do a lot of data entry and really need your numpad, I can heartily recommend this form factor. Most mechanical keyboards that are available in full size, also have a tenkeyless variant by the way.
As the name implies, 60% keyboards are ultra compact. They lack navigation and function keys that are found on a full size keyboard, but the functionality of those keys can be accessed via a second layer and an Fn modifier key. Some examples of 60% keyboards are the Happy Hacking Keyboard, the Poker II and the Ducky Mini. Given the fact that you are coming from a full size keyboard, I am hesitant to steer you towards a 60% keyboard.
Now once you have decided on a form factor, it's time to think about what kind of keyboard switch you like to type on. There are 3 major types of switches: the most common by far are Cherry MX switches. Less common and more expensive are Topre switches. Finally you have the classic buckling spring switch, as found on the Model M.
I'll start with the buckling spring. They are the grand daddy of mechanical switches. They were originally found in the iconic IBM Model M keyboard of the late 80s and early 90s. IBM has stopped making them long ago, but a company called Unicomp has acquired the patents and tooling, and they now produce Unicomp branded Model M's that are virtually replicas of the original IBM keyboard. This type of keyboard really is a typist's dream. Pressing the keys gives very solid tactile feedback and a loud (and I mean LOUD) thunky click. It sounds like a machine gun if you are typing on it at speed. If you share an office with other people, I would not recommend them. They are also not very good for gaming. This doesn't mean that you can't game with them, I have and a lot of people do, but other switch types just work better for that purpose.
Cherry MX switches are by far the most common. They come in many variants: linear, tactile, clicky, stiff, soft, ... The color of the switch indicates the type. For a first timer, I would recommend that you only look at MX Blue and MX Brown switches.
- MX Blue switches are clicky and give tactile feedback, a bit like buckling springs, but lighter and less loud. The same advantages and disadvantages apply: good for typing, bad for gaming, noisy.
- MX Brown switches are a good jack of all trades switch: they don't click, but they do give some tactile feedback in the form of a slight bump that you feel when you press down a key. I've never found noise to be an issue with them, they're effective to type on even if it's a bit less satisfactory than a clicky switch, and gaming works well too.
- MX Red is another common switch you find. They are a so called linear switch, which means that they are not tactile and not clicky. Pressing down a key feels just smooth until you hit the bottom. They are often mentioned as being the best for gaming, but they can be difficult to type on.
To summarize: when in doubt, choose MX Brown.
Topre switches are a bit of an odd duck. They are so called capacitive switches, a bit of a hybrid between rubber domes and mechanical switches. I could explain at length how they work, but that's not so important here. What's important is how they feel. They are not clicky, but they do give you tactile feedback in the form of a smooth bump when you press a key. It's a very different feeling than Cherry MX Brown switches though, the best I can describe it is that it feels less sharp and more rubbery, but still crisp. Bottoming out a key also makes a more muted "tock" sound instead of a harsher "clack". Most people who try them, really like them. They are also a very good all rounder switch, and are very satisfactory to use in both typing and gaming. The downsides are that not many manufacturers use Topre switches so you have less choice there, and that Topre keyboards can be quite expensive, think upwards of $200.
Now (finally!) let's go on to specific keyboards. If you want to go the old school way of buckling springs, you only have two options: get a Unicomp or find a used IBM Model M in decent condition. If you want to go the "mainstream" way of Cherry MX switches: Filco Majestouch 2 keyboards are about the best built keyboards I've ever used. Some other notable brands worth considering, but I don't have first hand experience with them, are off the top of my head: Ducky, KÃL, WASD and CODE. Razer and DAS aren't rated that highly, especially Razer since they use lower quality knock-off switches. For Topre switches, you really can't go wrong with a RealForce, and this would be my keyboard of choice if I had to choose one keyboard to rule them all.