I would say it depends on how long you've been gone and what you want to get out of it. I used to play from Classic to WotLK. I came back two days ago and I absolutely love it. Even had to start an account from scratch because I moved regions and it's fine.

You'll see a lot of players complaining about the scaling mechanic, but I think it's great! I really like the lower level zones, and now I can play multiple ones on one character without having to move to the appropriate zone. The pacing is so fluid and I really love the Cataclysm changes.

The built in questing interface is actually good now. Same for dungeon finder. I would say combat and particularly dungeons are a lot less challenging on the lowest difficulty now, but there's higher difficulties for that now. PUGs used to be a lot more social and slower paced, but you do get to see more content this way.

Some other things are a little cluttered. For example, some quests were removed, but the breadcrumb quests leading there are still in. The automated "squish" of 14 years worth of content also appears to not have scaled perfectly at every level. There's also some tinkering with classes and abilities that still needs a little polish. For example, one of my talents apparently needs me to double-wield, but the tooltip says "requires a one-hand weapon".

TLDR; if you haven't played for a few expansions, the zone scaling, streamlined quests/PUGS and the newly introduced zones and classes are fantastic. If you've not been away for long, some of the recently introduced bugs and changes might get on your nerves.