I agree that it is dictated by the story...
In <img src="/ubbthreads/images/graemlins/div.gif" alt="" /> you start as an unknown hero.... with a basically "save the realm" mission. You are a chosen one, so you have this ideal of becomming the hero and the divine one... Your only true enemy is the black ring. So you have a story with you as the good guy (weather you help the old lady over the street, or splash up mud at her is your choice)... This allows for a very "open" plot line... they can put quests of almost ANY nature into the story line.... the only guideline being you beating the black ring and becomming the divine one.
In <img src="/ubbthreads/images/graemlins/beyond.gif" alt="" /> however, you are a trapped prisoner. You "know" the history of the plot (DD and if you read the novella, you know even MORE of the history - how you got to be a trapped prisoner on an "unknown" dimension). Because you are a prisoner, there are only so many quests you can do.... its limited to mainly escape... with a couple of smaller quests to try and keep you going... After you escape from the prison, you find yourself trapped in a bigger prison.... Nemises. So you have yet MORE escape quests... I think that all the acts are equally interesting... the reason why most people say that act 4 is the most interesting, is that you can "see the light at the end of the tunnel" You are almost there.
IMHO the main reasons why people prefer DD to BD:
In DD you are
"free" and you are the hero in training.
THere are trees, greenery lovely landscapes (this has a major psycological effect).
You get to BEAT evil (well sort of, since you are fighting your failure in BD)
You finish as the HERO.
In BD you are
In prison, with an evil entity attached to you (even though Larian did a brilliant job of makign this FUN, there is still that lingering side effect of you are bonded with evil)
Nemises is a mainly GREY world.... make no mistake, a beautiful world.... but the lack of bright colours, trigger a condistion known as S.A.D. (Seasonal adjustment disorder). Even though there are no seasons involved in playing a game, the lack of bright colours (sunshine and green surroundings, or bright and happy environments), causes us to feel some how depressed to an extent. (Its called seasonal adjustment disorder, only because they first discovered it in the UK, during winter months when people became depressed, later on research and case studies showed that it is actually the absense of "happy colours and environments".
When you finally win in BD, it feels like you actually LOST... I mean... You just spent a fair amount of gaming hours, trying to escape , and then you discover that your alter ego is the BAD bloke, and he ESCAPED with your help!!!!
REasons why i think that BD is better than (or at least Equal to) DD
The game playability was improved a lot (they forgot to migrate a few small things, but mainly i thought that BD was easier to "click through" than DD.
IMHO the game was much better balanced than DD. I had to FIGHT my battles, with no aid of "quick kill tricks" like in DD... even though i exploited some of these in DD, they did act as a game spoiler (casting scorpion traps and drinking an invisibility potion etc.... ) In DD i also found that some monsters were way too easy, while others were just much too hard (for the XP you received or the importance of the monster in the story line)
The humour in BD was brilliant... (sarcasm, my favourite pastime)
BD was much more intense and much closer to the real way that "REAL LIFE RPG's are played. There are much more factors affecting your chances to do / receive damage etc...
There was this feel of "be bad to be good" about the game... i COULD be the real nasty thief / villian because i had no HERO reputation to uphold. So for those who like to play games evil, this was the perfect chance to do so....