1. You can store more items using a single-slot system. (it will fix muling)
This argument is completely faulty. There is absolutely no reason why a single-slot system would store more items than a grid system. Just think about it for a little while and it will make sense. The appropriate amount of storage space can be calculated and the grid size adjusted accordingly. You can either have 40 grid spaces that can hold various size items, or you can have 12 single slots. The "space" ends up being about the same, you will lose space if you carry all big items, but gain space if you carry all small items. Besides, a single-slot inventory wastes space as well. If a scroll takes the same space as body armor, carrying a bunch of SMALL items in a single-slot inventory would be wasting space. That is, if you had the grid system, carrying lots of smaller items would SAVE space as compared to a single-slot system.
2. Diablo 3 is about action, action, action!!!
This is a poor argument because that is not what the Diablo series stands for. Items were at the very heart of the game and served to make the game endlessly replayable. Arguably the biggest skill-involed aspect in D2 was choosing items. Increasing the "action" at the expense of item dynamics would be a terrible mistake. And what makes anybody think you can't have a grid inventory and still keep the action?
3. A grid inventory would slow down the action
Why? Because you have to stop and sort through your inventory? Well you are going to have to do that once your single-slot inventory fills up as well. No matter how big your inventory is, it will eventually fill up and you will need to sort through it. Yes, it is possible to get 6 8-slot items and then you would be forced to arrange your inventory sooner - BUT, you could also pick up 50 smaller items and not have to arrange your inventory for even longer. There is really no sizable difference between the grid system and single-slot system in this regard.
4. Managing your inventory is annoying. The game is not about managing your inventory.
Well I hate to break it to you people, but this has always been a key aspect in the Diablo games. If you don't believe me go read the Arreat Summit:
http://www.battle.net/diablo2exp/items/basics.shtml And just because YOU can't understand the importance it added, doesn't mean it wasn't a key factor that helped the games do well. I know I never had a problem with managing my inventory, and nobody I knew had any problem with it either - it was FUN and part of the game. In a game where items can make or break your character, the judgments made about which items to keep and trash end up being VERY important to how well you do. Item "size" has always been an extra factor that helps determine what to keep and what to drop.
5. The only reason anybody wants a grid inventory is because of artwork.
God, its really sad how many people are saying this. While the artwork IS a very important thing that adds to the "feel" of the game, the grid inventory does serve a purpose in ITEM MECHANICS. It adds another dimension for which you can judge how good an item is. When space becomes an issue, big items suddenly have to be a lot better to warrant carrying them. It's an extra dimension of realism that I feel is very important. Sure, you can use single-slot inventory and add a "weight" attribute, but why represent something numerically when you could do it visually?
6. Realism? The entire game is unrealistic, so screw your realism!
This is by far the most narrow sighted view and a complete fallacy. It's not about real-life realism. It's about RELATIVE realism. Having SOME realism is better than NO realism. When "realism" in this case, means you have another dimension by which to assess your items' value (size), a little extra realism is a good thing. Just because it's unrealistic for a people to shoot fireballs, doesn't mean Blizzard should say "oh well since the game is already unrealistic, we should have all the characters FLY and MORPH and NEVER DIE!!" No, of course not. That would be dumb. So is your argument against realism.
7. There is no reason to KEEP the grid inventory!
*sigh* This just means that the person couldn't go through the thread and pick out the many articulated reasons myself and others posted. This also completely ignores the fact that the grid inventory was in Diablo 1 and Diablo 2, games which were both extremely successful. YOU should need a reason to SCRAP it, not ask for reasons to KEEP it.