Speaking of walls of lava, your walls are very important when you are building aboveground. This is actually a completely separate concern from keeping your living space enclosed.
Basically, there are two important elements to your walls- security and aesthetics. Having your home completely enclosed does you no good if any mob or player can barge on through your wall, and having a tough wall is nice but nobody will care too much for it if it’s also ugly.
By far the harder is security, so we’ll tackle aesthetics first.
Generally speaking you want your wall to present a unified facade- that is to say, random spots of wooden planks or jungle tree in the middle of a wall of smooth stone or white wool is a rather ugly appearance, and will keep your home from looking like anything really.
Perhaps the best guide for this is to look at real-world buildings made of materials and with appearances that are available to you in Minecraft. Very few brick buildings have visible wooden beams, especially not one-meter-long ones. Nobody in their right state of mind would paint or plaster random patches of a wooden wall white, and slatted-side buildings look perfectly nice as long as there aren’t bare patches of stone or stucco in random spots.
Because of this, you’ll generally want to do a little planning for your walls beforehand. It doesn’t have to be much- just deciding on one or two materials and a set pattern for them is enough. The human eye looks for patterns in what it sees and when these patterns are broken up without presenting an alternitave, that will grab the attention. Too many broken-up spots and you have conflicting points of attention, which result in subconscious confusion about where to focus and what one is looking at- at best, this just looks ugly, and at worst it can actually cause headaches.
By the same rote, don’t do things like stick chunks of fence or such jutting off at random points- it looks nonsensical and disconcerting, and will make it seem like you have no idea what you are doing, even if you do know. There are several tried-and-true combinations, though you can certainly find your own- such as:
Birch logs (uncut) with cobble in between.
Oak logs (swamp color, preferably) with plank blocks in between.
Alternating logs of oak and birch.
Oak logs (grassland color) with white wool blocks in between.
Smooth stone blocks with cobble bordering on the windows and doors.
Solid logs (all of the same color).
Leaf blocks backed with dyed wool blocks in a color complementary to the shade of green the leaf blocks have.
With all of that in mind, you do still have to have security in mind. A lot of the more decorative blocks (wool and leaves especially) that make very pretty building surfaces are really fragile, and can even be accidentally broken as you go about your business in the area around your home.
There are a few ways to be secure in your home with the knowledge that your walls are difficult or nigh impossible to breach.
The most obvious way is to make them of an especially tough material. This can be really prohibitive, given the amount of resources and time consumed trying to construct a building entirely of obsidian. It also isn’t really a guarantee- anything you can place, someone else can break or even gather, depending on how things are set up on your server. It doesn’t help that the recent 1.2 update gave some mobs the ability to beat on and break down doors, which makes having a way out of your own house a bit tricker than most would like it to be.
There are other options though. Limiting approaches to your home by cutting out a big moat can work, as long as whoever you’re trying to keep out isn’t able to figure out how to dig underwater without drowning. Especially if you fill the moat with lava- but then again, a truly determined player will be able to figure out how to dig past underneath your moat and come up below your home. You can always make your walls thick and fill them with a core of lava, but this can be defeated as well through a variety of methods.
In the end, for dealing with other players you will mostly have to hope.
Mobs are another matter.
As long as your wall is at least two blocks tall, your pit at least two blocks deep, or your moat full of lava, you simply don’t have to worry about walking mobs like zombies, skeletons, and creepers. Endermen can only really be stopped by low ceilings. Spiders are slightly more tricky- aside the moat of lava, a simple defense won’t keep them out- you have to take advantage of their AI and abilities. Namely that they can climb vertical surfaces, but not the underside of a horizontal surface. Adding a lip to your wall, whether it’s a whole block or only part of a block wide, will not only stymie spiders but also often cause them to fall off your wall, injuring or even killing themselves.
On the upside, this also keeps your roof safe, so you can stand out on it and stargaze as long as there isn’t a skeleton on a nearby hilltop.