Okay, so in my opinion combat is amazing and much better than 2e for a couple of reasons. In the interest of fairness I’ll give some counter arguments.
In 2e I have to pass an attack roll, then the opponent may well parry or dodge, then I have to roll damage and penetrate armour and toughness. That’s three chances to whiff the roll and do nothing. But characters can over time get multiple attacks roll eventually to overcome this. So in 2e combat could sometimes take a long time. With occasional lucky breaks. IMHO it was the biggest flaw with 1st and 2e and the main reason I switched to D&D. Too many dice rolls to do too little.
4e resolves this by having one attack attack roll represent all your attacking for the round. Your opponent makes a defense roll (usually a melee weapon roll to represent parrying but could be dodge, or charm, or animal training or some other appropriate skill). The degree of success between those two rolls then is added to strength and a static weapon damage roll to calculate damage which is then soaked by toughness and armour. Hit location is identified by simply reversing the attack roll dice. You can still deal damage on a fail roll if your opponent fails worse. This takes all the whiff factor out of the game and massively reduces the number of rolls each round.
The maths of it becomes second nature very quickly and what I love most is that combat becomes less something that is done to you, but becomes a personal interaction that both player and DM are involved with. If you roll a crit on your defence roll you could also potentially hurt the foe. So it’s also an opportunity. My players love it.
They also love group advantage. Where a pool of points build that they can use between them to do extra stuff, tactics, boost rolls or get extra attacks. Because of the above an extra attack is a pretty rare and special. Not such a fan of core book advantage - but I’m currently playing and it requires individual tracking - I also think it has the ability to be gamed to do some pretty wierd stuff.
They aren’t so keen on the magic system where it is harder to cast spells than in 2e and it can sometimes take a couple of rounds to get a good spell off. Coming from D&D fire and forget wizards it’s a culture shock. I personally like that magic is harder. To be honest my player caster of which Ive got three have only just got to Tier 2 where the good spells come online. I’m also playing a wizard myself in an online campaign because I want to see how it feels to play them. 2e system was very simple and elegant. It worked, though I do think magic was a bit too reliable.
Some people don’t like the new session based meta-currency. I personally love it and I think the players do to. I think it’s mostly killer DMs who don’t because it gives players chances to avoid the way the dice fall.
Monster design requires a bit of thought. You need to take base creatures and then add templates and traits to them. The monsters in the bestiary as written will get… well… beasted. There are a couple of PDFs for improving them.
I think to get the most out of the game you need core book, Up In Arms, Winds of Magic and the Old World Bestiary. With these I think the game really comes into its own and is substantially better than 2e as several of the flaws in the CRB are resolved with those books.