Well, I dont want to write a long essay here about my experience with D2 but I will share some. These are just my opinion!
I played D2 with friends few times but it wasnt that much better than playing alone. On the contrary. Alone, I have all the time to collect the dropped stuff, go back, hurry forward if I want skip that part, etc. but in a team there needs to be cooperation and that seemd like
work to me. And to argue about a good item about who should get it with a friend is bad, too.
On a side note the patches rarely fixed important stuff. They added some new item types and features but most of they were still useless. I never found a good charm for example. And the unbalance between the characters is annoying. And the unbalance between the abilities of a single character are even more annoying. There arent many ways to play a characted but with 30 abilities there should have been.
I always liked the gems, collecting them, transmute them, but after the first act there is no reason to use them. A perfect gem is already useless in act 4 and how many do you get until then. Not to mention you need a socketed item for them.
In Titan Quest all is changed. The ability tree needs additional points to be able to choose higher ranked spells, like +5 point so they become available. In diablo you only needed the level. So can chose to have a L6 low power ability or a L1 high ranked ability, but not both. Unlocking the higher ranks is only worth it if you want to use all(many) abilities form that rank or else you are wasting point. So as many levels as many playing styles for a character. But wait. A character has not 1 but 2 classes. You get 1 at the start and choose another at lv10 I guess. with 2 points per level you can advance 1 class all out, or 1 here 1 there. Many possobilities and many combinations between the classes. 8 classes and you choose 2 - how is that?
And the gems (the had a different name). You dont nned the cube, they simply merge, and you dont need 3 for the next level. if you have the 5 parts it is perfect. Some need 3 part, some more. All items have 1 socket, so you can use your gems right away, and you can merge the additional part later, when u have 1 in the item already. Even rings have sockets.
And of course the abilities are all different from D2, and there are some I never saw anywhere before.
That's a long enough essay I guess