First of all: the boots are the most important equipment for snowboarding! It's good to read something about it but please go to a real board-shop before purchasing them! Choose one of those fancy ones where guys hang around listening to punk/rap-music :)

The common ski- or sport-shops are in general not that reliable when it comes to snowboarding.
The other considerations are summed up in this article from the Austrian board-shop "Blue Tomato":
Most important: the size
The most important criterion when buying a new
boot is the right size. The Mondo point size, the length of your foot,
is a good guideline. When your legs are straight, you may touch the
front of your boot lightly. However, when you bend your knees this
should not happen. Pay attention that your heel is snug in the boot
and cannot move. Boots of different brands do not have the same length
and width, so keep in mind that the boot should also fit your binding.
Some brands use half-size liners in their outer shoes. If you have
large feet but do not want to ride a wide board, you can save
important millimetres in toe- and heel area with this information.
The flex value
Snowboard boots are available as hard- and soft boots. For freestyle
as well as freeride, you use soft boots. They are available in
different flex rates: softer boots for freestyle and harder boots for
freeride. Hard boots are solely for racing and carving. They greatly
restrict the freedom of movement, so that you can bring pressure to
the edge, even at high speed.
As you've described that you want to ride on the slope and a little backcountry, I would choose a flex value of 7.
Lacing
The lacing of your snowboard boots is essential for the hold. As a
rule, you can distiguish three different lacing systems: traditional
lacing, quick lacing systems and Boa. So the question is: what is the
perfect system for you? We will explain this!
The classic among lacing systems is still the first choice for many
riders. No other system can adjust the pressure as well as with the
old-school variant. The laces have a solid core that prevents the knot
from loosening. The soft coat gives the necessary grip to your fingers
to get a bombproof hold. Okay, if you ride really hard, you’ll
probably have to readjust them during the day.
The quick lacing systems, speed lacing, have become a fixed part of
the sport. Nowadays there is an almost bewildering array, from “twin
lacing” to “speed zone”. The principle is the same for all of them,
but, depending on the price range, you get a system for the whole
shoe, two divided speed-lacing systems for the top and bottom of the
boot or even a three-part system with extra lacing for more heel-hold.
With these systems, you thread the laces through plastic- or metal
devices, tighten them to the desired level and secure them in the
device. This only takes a few seconds. The more zones the system has,
the more accurately you can adjust the boot to your foot and
preferences.
Superficially, BOA is a quick lacing system. The only difference is
that it uses wire and wheels. Depending on the price range the BOA
system also offers, one, two or even three adjuster wheels for the
different areas of the boot and often even for the liner. Turning the
wheel tightens the bootlaces evenly; pushing the button loosens the
wire again. You can also use this system easily while wearing gloves.
When it comes to hold, BOA is the first choice. It is not the fastest
system to tighten, but it is number one to loosen: one button-push and
the wire is loosened!
A few words on the Liner
The liner is included with your snowboard boots. It increases the
comfort of the fairly hard outer boot. Several models even have a
thermo liner, which you can heat-mould to match the shape of your
foot. If you are prone to pressure points or particular parts of your
feet give you problems, a thermo liner may be the solution. You can
get your thermo liners customised in many Blue Tomato shops; with some
liners, you can even do it yourself in the oven at home. There are
liners, especially for the park, with integrated cushioning that
dampens hard landings.
Sole Cushioning
Boots have cushioning so you can survive hard landings without
bruising your heels. Essentially, there are four types of cushioning:
EVA, an especially light and flexible plastic that is often used for
cushioning. However, it loses its shape with excessive cold, and
therefore diminishes in quality. PU is a strong and durable plastic
that does not lose its properties, even in icy temperatures, but costs
more than EVA. Air cushioning in the sole absorbs high impacts. This
works especially well on the heel and forefoot areas, but less with
icy temperatures. Gel pads are usually incorporated around the heel
and work in the same way as air cushions, but are less susceptible to
different temperatures.
Don't forget the socks
Even the perfect soft boot cannot realise its potential when you do
not wear the right socks. Functional socks, which are long with
reinforcements at critical points, maximise the fit of your shoe.
Normal socks are too thin, can chafe at critical points and, in the
worst case, slide down, wrinkle and provide unnecessary pressure
points that can become really painful. An adjustment of the liner by
the use of special fans and sometimes even ovens is possible and
advisable for most models in the middle and upper price segment. You
can find information about this in the product descriptions or on the
boot itself. In case of doubt, our customer service will help you.