"can you elaborate a bit on what you mean by it feels like Diablo now? Do you mean the pace of combat and whatnot? How are the instances different? What hours did you put into those leveling sessions?"
There are more than several tweaks to combat, gameplay and the interface. Gameplay is much quicker - players seem to do more damage and get effective exp for kills. Also, the design of some of the classic instances helps the experience have a faster feel as the designers cut out a lot of the fat content no one ever utilized and streamlined the process with quests inside the instance entrance and also being about to complete quests right after you kill a boss and begin the next quest without having to go back. This is not true for all of them - I ran Blackrock Depths and it seemed pretty much unchanged although there were 4 quest-givers at the entrance - I still had to run back to them later on (but it did go by MUCH quicker). Boss fights in some of the early level instances were pretty awesome - Deadmines comes to mind for sure. They cut out a lot of the crap level design, even walking to the ship and fighting up to the top of the ship is much much shorter. Both the boss and trash fights are fun and there are new bosses that actually have some pretty good moves (you can run Deadmines in like 15 minutes now I think I ran that instance 4 or 5 times. Needless to say I was disappointed when I got to level 20 and could no longer go there (it's a lot of fun).
At this point I'm all about the streamlining of WoW - half the time I auto-run - but no you can even click on a far away NPC and the character will run to the character to turn in the quest without having to sit there and physically do it. They cut out the bullshit with the talent-trees and they pretty much hold your balls through the process (to piss off all of those mathematical geniuses out there), got rid of many bullshit abilities. When you "ding" the screen will display what is now available, if you have new skills at the trainer or not and if you received a talent point. I definitely don't feel like my Hunter's rotation feels as powerful as before but I still welcomed the change to figure it out all over again (lot's of aimed shots to increase focus). I trust Blizzard knows what they are doing for the overall enjoyment of the game so I don't complain like a lot of annoying assholes out there.
A lot of the fat quests - gone. The zones I mentioned - Blasted Lands and Swamp of Sorrows have been streamlined to an effective degree. The quests are more interesting and less grindy. Most of the time they ask for #X and #Y you will find #x on each kill instead of having to grind an area forever waiting for drops. Saves lots of time and hassle. Each quest giver gives about two quests - you finish those two quests and it opens two or four more (with the addition of another questgiver). With less quests the rewards are bigger - you level 25% quicker and the quests more interesting. Not to mention that many of the new enemies in the changed classic zones are pretty badass and the new cosmetic changes are nice. Blasted Lands has a couple of new areas - a Tainted Forest (with some BC creatures in it) (that is currently under-utilized) and a shoreline where Murlocs are being used to cart treasure to a cave as slaves. Once you finish the quests in an area you might be cannonballed to the next zone to begin a new questline there. You will be flying through the air (without a mount) in this one.
With my rogue I can pretty much two shot any non-instance mob. It's easy leveling - but supposedly the combat in the instances will require more attention to CC & threat as it used to be while the combat is much more intense & fast paced.
I spent about 20 hours total playing this weekend - I remember it used to take about 2-3 hours to level - I gained 8 levels on a newb rogue and 8 hours on my hunter. You do the math. Once I started running Deadmines I gained three levels in a very short time. So, you can either choose to see new quest content (which contains some good stuff) or you can choose to powerlevel in the instances (which are awesome). I like the new areas, the new quests but F-it - I'd rather just keep running these changed instances.
I've only played little bits here and there so I've only touched the tip of the iceberg I'm sure- but most changes I've seen have been welcomed - to F&^King awesome.