From what I remember, it was basically Lego Digital Designer but with every lego piece ever made being available.
Right, so let's go over what the building of LEGO Universe was in comparison to LEGO Digital Designer.
LEGO Universe building contained what is certainly a direct port of LDD into LEGO Universe, as a tool to add models to a variety of small isolated areas called properties. With LEGO Universe, you could build a model, and have it converted to a somewhat optimized model that you can place on your property or in your inventory. Properties would contain one or more models and these models could be given programmable behaviors to move, break, speak, spawn enemies/powerups, etc.
Except the behaviors had rather broken netcode with regards to their behavior and physics, and on properties that were moderately demanding or more, they would more than likely break, stutter violently, or become utterly unresponsive.
So, back to the port of LDD. At around LEGO Universe's launch, LDD had been given the same parts library as LU, so with respect to the parts inventory the two were the same. LEGO Universe's brick building also handled essentially the same way as LDD with some rather intuitive clicking, dragging, and rotating, though LU building was missing the ability to rotate bricks to a specific angle, so anyone trying to get mathematically fancy with hinges were in for some difficulties. Additionally, LEGO Universe had completely custom UI elements that served a similar purpose to LDD's building. There was a color-picker that didn't have labeled colors which was terribly unfortunate for any colorblind players. There was a bottom bar to select brick selection/movement actions, nothing interesting there. And then there was the parts inventory.
Oh boooooooy, the parts inventory.
Unlike LDD, LEGO Universe's inventory wasn't in a nice and orderly order. LDD had parts sorted by a category and then ordered by size, and any difficult to find brick could eventually be found with a quick online search for a part number and the use of LDD's search bar. LEGO Universe building had no search bar, a brick order that was disorganized and determined by the order in which you acquired your bricks, and sorting by categories involved combining one or more categories as a filter, which was an interesting feature, though it was a bit hindered by the fact that some bricks were placed in questionable or outright incorrect categories, and trying to find a brick could take minutes or forever, because as a player tried to find a brick in their inventory, it could be very possible that the brick wasn't in their inventory at all.
Because in LEGO Universe a player had a limited number of bricks, and if that number was zero, that brick would not appear in their inventory at all.
This was the most infuriating thing about LEGO Universe. Parts had to be acquired either as random drops from enemies or by purchasing them from vendors scattered across the game. Which vendors had which bricks? Well, other than vague categories, it wasn't well documented, and a builder could find themselves wandering about the game, wasting their time, hopping from inventory to inventory, through many long loading screens to hunt down that one brick they needed, and that simply was not fun.
There were additional complications to this limitation of the player's parts inventory. Say, a player was building a grand cathedral, with a spectacular line of flying buttresses. The player could and would build one buttress and attempt to clone it. But wait, what if the player didn't have enough of a certain brick to clone the buttress! Well the game would fail to clone to model and tell you that you don't have enough of a specific part. Which part? Excellent question! The game would not tell! If a brick was missing, the player would have to go through the model, select a brick, manually find that brick in their inventory, count the number of that brick is in the clone target and see if there's enough, and repeat this process for every brick until the culprit was found. Then go vendor hunting again, get more of the brick, and return, only to discover that the model still can't be cloned because, gasp, there's another lacking brick that is preventing cloning!
Incidentally, models magically disappearing from a property, usually during a crash, was a common occurrence.
Incidentally, Incidentally, placing models themselves could be a nasty pain in the bum-bum. To move a model, the player could walk near it, and magically pick it up and carry it around with a convenient shadow to indicate where it will rest on the ground. If the player wanted to place their model in the air or on top of another model, the player would have to go into build mode, place some bricks, put the model on top, and remove those bricks, though it's recommended to leave at least one brick on the bottom, otherwise accidentally picking up the model would cause it to return to the ground with no option to return it to its original position.
Hmm.
Did I mention that your models could only contain 1500 bricks?
Did I mention that you could only have 1500 bricks active in build-mode?
Did I mention that you could not duplicate a model outside of build-mode?
Did I mention that this means that to duplicate a model it had to be split into two parts, which then had to be cloned, and then connected together again with an additional waste of time?
A waste of a lot of time.
A waste of a lot of
my time.