in terms of buying a cab, what specs do people consider to be important?
Sound, Volume, Weight and Size are the specs that most people are looking at.
When in the market, you should answer a few questions that will help you look into the right cab(s).
1. How many cabs:
Will one cab be enough for everything you do? If that cab has more than one speaker, might it be a good idea to get two cabs (i.e. instead of a 410 you get 2 210s or instead of a 210 you get 2 110s)?
You should have a cab that can handle all your volume requirements. If there is a big gap between your loudest requirements and your standard requirement, it could be that you can get away with one cab on most occasions and only need to bring the other one if it gets loud.
2. How loud should it get? You don't bring a knife to a gunfight. You also don't bring a 2x10 cab when the guitars are playing 100W tube heads into 412 Marshall cabs and the drummer is playing those sticks that don't get thinner at the end because these break all the time.
At the same time do you not really want to bring a 8x10 to the ukulele group. It's overkill if you have to sell the compact car and buy a truck to transport your full stack when all you do is folk and jazz gigs.
Your cabinet should be loud enough so you get heard and have a bit of room to spare, but at the same time it should not nuke the entire ensemble when turn the master vol to 3 instead of 0,5.
3. Is size and weight important? You can pick any two of Hoffmann's three: Light, Loud, Cheap. There are cabs that are light and loud, but they cost you. There are cabs that will be loud and are affordable, but you better have a healthy back. And there are cabs that are light and cheap, but you need to bring a lot of them because they are not loud.
4. Ohms. On a solid state amp you must not undercut the minimum ohms requirement. With a tube amp you should match it exactly, even though there is a certain margin, depending on the amp.
If you're planning something modular like a 212 cab and a matching 112 cab, you'll typically end up with something like an 8 ohms 112 and a 4 ohms 212, which is good because the power distribution between those three speakers is even - but the resulting load is 2,67 ohms which requires an amp that can do 2 ohms and there are not that many of them out there. If you're planning on maybe adding a cab in the future, go with the 8 ohms version if you have the choice (unless your amp can do 2 ohms).
