Depending what sources you look into, Landmines do exist, and in a powerful format in Star Wars, but of course none are seen on film. Considering that Landmines are considered one of the deadliest forms of combat dating back to the late 19th century and on into today I think it's reasonable to assume they exist in Star Wars though, and can be effective.
I know the old CCG's actually visualized Landmines in Star Wars, the Jedi Knight/Dark Forces series show a variety of types of mines both proximity and self-detonated, and other WEG claimed them as well I think.
As for obstacles, I ask myself how effective they are against something as powerful as an AT-AT... Also, perhaps obstacles of some sort were set up and AT-ST's were able to clear them or warn the AT-AT's ahead of time since at least one ST was seen advancing ahead of the AT's... Perhaps that's its goal? Kind of like the Speederbikes (if we accept they're there).
While I agree about the use of cannons being that you want to penetrate armor as quickly as possible as well, in Star Wars, the AT-AT may just prove too "advanced" and require multiple hits as well... That or climate (and the need to adapt weaponry to it) as well as the limited time maybe didn't allow heavier artillery batteries to be set up? Some EU does say there's very similarly designed weapons to the ones at the trenches, but much larger and powerful... Force Commander showed Radar/Dish laser turrets that are stationary similar to the anti-infantry PD Towers that were in place. Maybe the Alliance hadn't adapted the larger artillery then yet?
The planetary shielding issue though is a whole other can of worms, but I think it's explainable... The properties and specifics of shielding in general, and the way the Rebels shield itself worked seem to be the keys to understand how the assault unfolded.
We know the shield DOES need dropped, as Leia says so, but a drop in the shield seems "minimal" from what she says and they probably are coordinating the Transport's exit with the shield's momentary drop and reinstatement... If that's the case, bombardment wouldn't be efficient at all.
Also, the Ion cannon doesn't necessarily need a shield opening to fire... At least the film doesn't mention it, so it's possible the Ioncannon fires outside the shields. Since Y-Wings and X-Wings are shielded but capable of firing OUT of their own shielding I think this is reasonable to assume it works similarly. Can't shoot IN, but can shoot OUT.
As far as weakening the shields by constant bombardment... I don't see a reason to doubt Veers' words that the shield would deflect "any bombardment". Maybe it's a slight exaggeration, but perhaps in the allotted timeframe they have to assault the base and stop the escape while the vessels and people are STILL within the atmosphere or on the ground then possibly he was speaking in the short term rather than if they simply took orbit and slammed the shields for hours or even days till it broke.
If the shields ARE really strong, we also know they have their OWN shield generator too. The shield generator is referred to as just that, not a "generator" generic, but a shield generator, and when it's lost the base seems to still have power itself (and EU claims the Ioncannon is still firing till the bitter end too). With that in mind, the generator isn't part of the rest of the base's energy distribution. it could then theoretically hold for the duration of a prolonged seige even.
It explains the importance of swiftly getting within range and destroying the Generator, thus allowing TIE's to swoop in on the rally points and hangar exits while troops deploy to sweep and clear the base itself.
Again though, for exiting transports it does open and I suppose shots could come in, but if the opening is coordinated well it's so brief that it could really be a crapshoot if the Empire even got one fighter or orbital shot in with the brief opening. Also, does the ENTIRE shield shut down for a brief moment, or only a small box, possibly only large enough for a vessel or two to exit? That's another good question to ask when determining the physical properties of the shield itself.
With that in mind, and judging by what we KNOW of the military assault and defense shown in the film, it seems to me that the shield's a very complex and customizable resource that the Empire basically sees as hopeless to combat itself... From what I can gather they'd hoped to hyper in a distance away and drop in fighter squads as fast as possible till the Alliance realized what happened (Imagine when they said "Oh crap", and flipped the "on" switch to the shields and the attacking TIEs that hadn't made it yet slammed them? Yikes).
Makes you wonder what WOULD have happened had Ozzel not F'd up. LOTS of Rebel losses, and likely a heavy space battle, though not in space but rather just in the atmosphere... X-Wings, Y-Wings, and TIE's duking it out while TIE B's go and blow the bejesus out of the shields allowing some heavy-duty troop landings. The Rebels would've been F'd pretty bad at that point.