Wouldn't surprise me if they'd seen it on the map, but they simply dismissed it as a tough place to potentially break into versus all the houses in the area (which may have been the right thing to do, since they've obviously survived all winter and don't seem much worse for wear). The dialogue doesn't strictly rule that out: Daryl spots it, Rick walks up, and then the light goes off in his head. Could also be that on seeing the gated field that they realized the clearing it was do-able, and that getting from there into the prison was a similarly achievable goal.
I loved this opener - it had plenty of zombie smashing, though one reviewer pointed out that it may signify even more episodes without so much as a moan from any undead. I do also question the tactics, but I figure the writers hadn't fully realized what the prison would look like on TV. In the comics, the prison was much larger (even what we see in this opener is visibly smaller than the prison visual from last season) and aside from the tall fences, it was more walls than anything else. They may have visualized it differently than what we saw.
That said, it should have been easy to take care of the walkers at the fence and then send scouts into each area. Once a group of zombies was discovered, you'd retreat and agree on the best way to taunt them, even a few at a time, back to the fence and take care of them there until the area was clear. However, I can imagine that the writers would want to skip to the insid of the prison right away and thus skipped over some of this (and show our heroes as actual zombie-killin' HEROES for a change, with an appropriate undead bloodbath).
I can't remember if they figured out how some of the zombies had actually got that way without being bitten? There were a couple security guards last year who turned without having dental impressions made upon them, and now there are some prison guards who apparently zombified that way too. Perhaps they had been lightly bitten somewhere else, armored up, and died later? Opted out with poison? Starved, as several inmates seem to have done? We'll probably never know. Still sucks to have your face ripped off with your helmet later, though.
As for the characters, I loved 'em the first season and their unending bickering on the farm last year got grating really fast. As expensive as it is, this show needs to have more kinetic force behind it to keep it going. We all *like* these characters, and we arguably still do; we just want them to do more so we can continue sympathizing with them.