In my experience, it's only as complicated as you make it. I play in one campaign where the rules are so strictly enforced that gameplay suffers terribly, and there is virtually no room for house rulings or leeway in any situation. This DM ran us through (which one, the Forge of Fury?), and still considered the Roper across a 10-foot underground river to be EL 10 (Its base CR), even though it could reach us anywhere in the room with its tentacles, while we could do nothing but fire ineffectually without jumping across, risking being swept away to certain death. It won initiative, dragged the tank across the water, and nobody could even get near the thing to free the tank from the Improved Grab (read "broken ability"). Luckily, the Bard, in his single wisest play decision ever (Wis 9, complete buffoon) attempted to bargain with the thing and succeeded. The DM's argument that the EL wasn't ridiculously high was that "well, you could bargain with it, so that brings it back down", which she cited as an option suggested in the adventure text. The problem with this standpoint, IMO, is that this option should already be factored into the CR of the Roper, as its availability is not restricted to the confines of that specific situation. Therefore, I argued, the EL should be increased due to the large terrain advantage in favor of the Roper. It's just very difficult to get some people to improvise, even though guidelines are set out in the books, and it is only fair to adjust certain rules as the circumstances demand.
Anyway, in this campaign, even though most of the players are fairly rules-savvy, we often find ourselves having to backtrack when we later realize that a given action couldn't have been taken, especially with regard to monsters, as it can be difficult to ascertain every aspect of a given monster's abilities on the fly. The rules, however, must be followed verbatim in this game, and it drags things out and causes a lot of hard feelings among the play group, especially between this DM and myself, as I am quick to point out when I have been slighted by an "illegal" action on the part of a monster, or the (perceived) objectively illogical nature of a given rule. I have been justified many times over, as several of my suggestions were actually implemented in the 3.5 conversion, those same suggestions which had previously caused hours of heated debate. Conversely, I also point out when players declare "illegal" actions, etc., as that's just the way this campaign has been run, and I prefer not to "cheat". To this DM's credit, however, she is extremely adept at roleplaying NPCs, and there are upsides to ensuring that the rules are followed. I enjoy her game immensely despite my gripes with certain aspects of it.
On the other hand, I have played in campaigns where the DM was very good at improvisation on the fly and very reasonable about tweaking rules for the sake of fun. In my opinion, open-mindedness is the only way to approach the role of the Dungeon Master when your goal is to provide the most enjoyable game possible on all sides of the table. Some people will surely disagree, but I can tell you that from my experience (about 13 years), I have found the most fun in games where the DM was less of a rules cop (especially with one particular DM- amazingly creative and animated), and I feel that this statement would be supported by everyone in my play group who has participated in any of his major campaigns.