Beyond Divinity is flatter than D2:ED. Divine Divinity, the first in the series, is a 2D game (isometric, fixed camera), with handy-drawn animations for the characters; Beyond Divinity used much of the same game engine, with 2D backgrounds and 3D character models (so had more variety in number and appearance of equipment and characters).
You could say Divine Divinity is similar to Diablo, but with good quest and story design. Of course the quest design is most elaborate in D2:ED, but DD did have a few quests you could solve in different ways, and mutually exclusive quests where you needed to choose sides.

Beyond Divinity is divided into 4 acts, so like D2:ED there are points where you can not go back (the first act was designed to be relatively linear, but it opens up towards the end, and later acts have much more freedom of movement). Divine Divinity was more open, with most of the world available most of the time (there are of course quest opened locations, etc, and near the end of the game you take a one way trip to the final area).
BD has summoning dolls you can acquire (one in each act), similar to D2:ED's creature, that you can control directly to help in combat (though even upgraded they are not very strong), or simply use to carry loot. There is a Battlefield in each act, which contains merchants and some randomly generated dungeons (easy access to merchants is handy, but I never bothered with the optional, hack-and-slash based dungeons).

DD is a single character game, like D2:ED, with the ability to summon creatures or resurrect fallen opponents to help you. BD has 2 main characters, though most of the time you can have them both selected, and direct them to move or attack as one (whichever is not in the lead can be set to automatically attack, which is handy for an archer; you don't necessarily want a melee warrior rushing off to fight without the other character).

The music is similar in all 3 games, as is the humour. They stepped up the humour a bit in D2:ED, but I think DD has the edge for easter eggs (references to various games, books, etc).

In DD and BD the inventory is weight based, so how much you can carry is based on your strength (in BD you can just overload a summoning doll, as long as you don't need it to be able to move). Unlike in D2:ED, you can use / move / organize any chests you want to stash stuff in until later.

The 168MB demo for Beyond Divinity can be downloaded from GamersHell, or some of the links here might still be valid.
The 400MB demo for Divine Divinity is available from Fileshack, etc. The beginning of DD (the area in the demo) and the very end of the game are heavy on the hack and slash (most of the rest of the game I could switch between exploring / fighting and non-combat area as my mood suited), but the demo should still give you a fair idea of the rest of the game.

Quoting myself from a couple old topics, to avoid spoilers:

 Originally Posted By: Raze
I think DD is generally considered to be better than BD, though of course individual opinions vary. There are some aspects I liked better in BD, and some things I liked better in DD.
There is no BF or random quests in DD, but there is more of an open world, with lots of places to explore. I don't know if there is a huge difference in total size between games, but DD feels more open and immersive, with access to most of the gameworld from the start.
There is a single character in DD and the skill system is different, but the music, writing, quests, etc are similar. There are a few quests where you can choose the solution, but not as much as in BD. Stealing is much better in DD, as NPCs will react if they see you, and their opinion drops accordingly; some NPCs are very touchy about their stuff, and react badly even if you just move something, or try to get into a back room.
DD can have a few challenging parts, especially at the beginning. It tends to get easier as it progresses, though. There are exploits and effective skills in both games to make fights easier, but overall I'd say DD has more options for dealing with difficult parts, and if things start to get too easy you can always choose a handicap or increase the difficulty setting (I bought and saved charms, but by the time I had equipment good enough to use them on, I didn't need the extra bonus). I re-loaded too much to make sure I got the best equipment, though, so the relative ease of the end game was partly my fault.

 Originally Posted By: Raze
Btw, why do you think there was such a huge backlash for BD? Was it 'cos of the plot or the gameplay?

I don't know if I'd call it a huge backlash, but what there was was mostly due to changes made in BD and copy protection. While there were valid complaints on both counts, the first was mostly people not giving the new design a chance and the second was primarily hearsay and bad PR.

The separate acts do drop the level of freedom and feeling of immersion, and starting out in a prison there are not a lot of quest opportunities, so there is nothing to let the player know that if they get through the initial linear area (designed in part to guide new players and let people get use to the controls) that they will be rewarded with actual quests and more freedom.
I played Diablo and got bored with it. Playing the DD demo it had similar controls, but exploring around Aleroth it was much more immersive and there was an actual plot and real quests. While the game looked like a good buy before this, I knew I had to have it after meeting the existential skeletons in the catacombs. It was a shame BD didn't have a moment like that escaping the prison.

With the skill system there were a few relatively minor problems with the design and implementation (ie skills you already knew at or above what was being offered were not indicated), but many of the complaints were from people just upset it was different from DD, who did not actually try using the system. I could list some skills that IMO should have been designed differently, but there are also good changes made allowing more flexibility and customization is spells.
Days after the release some people were complaining that there were too many skills and far too few skill points to go around. I had been holding off using a lot of skill points until then because I was thinking of eventually changing weapons, but with the complaints I decided to start hoarding them, and had plenty of spare skill points pretty much the entire game.

The only problem with learning skills from a teacher in BD is that if you pass whoever offers the skill you might not be able to get back if you discover that it would come in handy (though the BF merchants also offer a lot of skills). With DD2 presumably being a seamless world, this problem would not exist.
Learning from teachers allows the skill system to be connected more with the game world, and is more realistic. There can be unique skills only available from certain quests, or from certain NPCs involved in various moral dilemmas, etc, which would add to the replayability of the game.

When BD was in development, Starforce was suppose to be the next great thing in copy protection. The fact that some people have had problems with it was, from what I saw, completely blown out of proportion. Of all the startup problems and conflicts with BD (which effected a minority of players) a small minority were due to Starforce, and in those cases many could be fixed with updated drivers or a workaround from Larian. A lot of the anti-Starforce sentiment seemed to be based on rumour and bad PR. The way some people reacted you'd think Starforce had destroyed their computer and then kicked their puppy, but in a brief search at the time, I found many more vague third hand references than actual reports of either hardware or serious software problems.

I hope that if Larian can afford it, that they will go for superior voiceactors like the ones in Arx Fatalis

After the demo was released there was a large, very vocal group complaining about the voices, particularly the DK. The DK's voice wasn't what I was expecting, and I didn't particularly like it at first, but it certainly wasn't horrible. Larian hired different professional voice actors to redo the voices (delaying the game release and costing more money). After this, a previously mostly-silent group complained that the original voices were better. I don't recall the demo DK's voice very well, but the skull's voice was certainly much better in the demo. I'm not really picky about voices, actually, and found the BD's voiceacting acceptable to good in most places (at least good enough not to draw attention to itself, anyway).