I like a flat-plate style side rail, which gives a lot of options for bolting things directly on without funky brackets or having to double-hole tubing. Plus you can put the lights right into the rail cutouts. Mine has square cross-tubes but flat sides and a metal fairing that helps with structure at the front. I also have an opening at the sunroof so I can just climb up easily through there with no need for the weight of a ladder (or I can climb up from the rear tire, no big deal). I don't carry much of there since this truck is huge anyhow, so it's usually empty (read: less wind noise/resistance/weight up high) other than the awning and water tank. But it's available if i need to throw something up there or whatever. So basically I use it for carrying stuff that needs to be up high (awning/water/lighting) and pretty much nothing else.
It has hidden tiedowns so less wind noise, and the composite chicken coop flooring really reduces wind noise over the bars and makes for a strong and lightweight platform.
Don't: Don't bolt it up there after one coat of paint if it's steel. Else you'll be chasing down little rust spots forever, which will inveitably drip down the sides of the vehicle. Paint it multiple coats with a good, thick, weatherproof paint. Things on the roof can get rusty and ratty-looking quick. Ask me how I know
Don't: Don't direct-wire everything. Have a junction box up there for lighting, etc. so you can disconnect things if the rack has to come off.
Do: if you use tubing with sealed ends, make sure you run a coat of seam sealer around ALL welds so water can't get into any little gaps in the welds, etc.
Do: keep it as low to the roof as possible. Just far enough so with a heavy load it wont' flex or touch the roof. At the roof peak in the center mine is about 1" clearance (more on the sides).
Don't: invest in high-dollar light bar or side lighting on the roof. I find that I rarely use mine (cheap ebay one) and when I do, I'm usually going slow offroad so the cheap ones are just fine. High-end ones with long-range optics are kind of pointless for slow-speed stuff (we use high-end ones for stage rally, but we're going 80mph in the woods at night and need the range). I totally don't get people who buy $500 light bars for the roof, unless they're desert-running or something. Just my opinion though. Maybe some people need them for some reason.
Do: if possible, take off your factory rack and build feet to bolt directly into the factory roof mounts. Secures it better and allows it to sit lower on the roof than mounting across factory crossbars or whatever.
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on my "fun" rig I have a lighter-weight rack I built that basically holds a smaller awning, some lights, and a rifle case that I got to fit my traction boards since I prefer to have them stowed away out of sight but dont' want to put muddy stuff inside.
the forward extensions are for branch risers, though admittedly the places I go I rarely attach them since it's not usually heavy scrub.
