This next text will be about 40k and Warmachine. However, it will NOT be a rant how much GW sucks or anything like that, it will be a text from a 40k veteran that has expanded to Warmachine. I will not talk about Games Workshop as a business, only about the games. I think it will be useful for people who are familiar with 40k and are thinking about trying Warmachine. I like 40k ,but Warmachine might be a better fit for me and I will discuss why.
First difference I would like to discuss is scale. In a typical 40k game you are going to have a LOT more models than in a typical Warmachine game. Only a rare few non-vehicle models work as individuals in 40k. Most of the time even those will join a unit! A unit of 10 models is not a large unit and especially in the newest edition, there will be a lot of vehicles on the board. But Warmachine is skirmish scale. There are units, but even in units every model will function as a individual. Every model can shoot at it's own target and hit's another model, not a unit. Every model will also roll it's attacks separately. There will be a lot of models that are purchased alone, which are called solos. Also, full units are quite pricy, so list with three full units of 10 models are quite rare in a normal sized game. It's possible and can work quite nicely too, but it's rare. Because every model is treated this way and every attack is rolled separately, Warmachine is not a good game for you if you want to see two huge armies clash together. But if you want more detailed rules for every model and like smaller scale games, it's perfect for you.
This difference in scale means two additional things that are not directly connected to the rules. It's far cheaper to buy a 'normal' sized army for Warmachine. It's also good news to slow painters. It's actually possible for me to get my army fully painted. The downside is that you won't get to play with huge armies. Privateer Press has published rules addition called "Unbound" which let's you play larger armies more effectively. However, it's not a real expansion like Apocalypse for 40k and you still won't reach even near to the scale of Apocalypse. I like the skirmish scale a lot, but it's up to you to decide whether you like it.
The way the lists are build are completely different too. In 40k, you decide your army and build an army from a codex. Every unit is categorized and every category has a limit of how many units of that type you can get. Also, you have to get two troops and one HQ. In Warmachine, you decide a 'caster. Then you can get any unit/solo/warjack of that casters faction and any mercenary unit that works for that faction. You have to take a Warcaster and that Warcaster grants a certain amount of Warjack points that you can only use on Warjacks. You won't actually pay for a Warcaster, but stronger 'casters will provide less warjack points. Every model has a FA, which determines how many of that unit you can take. There is a lot more freedom for list building in Warmachine. Only thing you MUST buy is a Warcaster and only the faction and FA limit your choices. There are faction books which include most stats and background for your faction, but these are not the same as codexes of 40k. You don't actually have to buy this, as the stats and special rules for a model comes with the miniature. New units get revealed all the time, for all of the factions and you can add them to your list immediately.
This has the good side, that new solos, units, warjacks and even warcasters arrive on a steady pace. You also have a huge amount of options for list building. But it has a downside too. Rules for the models are updated very slowly. When the game changed from mk1 to mk2 every unit got new mk2 rules. But since then, rules for the old mk2 models have stayed the same. It's going to be a long time before they publish mk3, so you are stuck with the same rules on a model for a significant time. This is mitigated somewhat by the fact that (at least in my opinion) Privateer Press has done a better job at balancing the rules. There are few units that are considered very powerful, (banethrall bomb, eGaspy and Reckoner comes to mind) but there are far less models that plain suck. Most models have at least one Warcaster or solo that makes them work.
That brings me to the next difference, Warcasters. These are way different from your HQ models from 40k. In 40k, you need to bring one and they are powerful, but losing your leader is not catastrophic. Warmachine games are won by either scenario objective or 'caster kill. It's literally a lost game when you lose your Warcaster. Warcasters are not only leaders of your army, they are controllers of Warjacks. Warjacks are steampowered robots that are controlled by accessing their magical cortexes. This is where the Warcasters come in, they are people with magical skills who can control these metal beasts. If you assume they are all stereotypical DnD mages, look at the Butcher from Khador and think again.
You have to base your army around the warcaster. It would be silly to pick only shooting models for a warcaster that boosts melee efficiency of your army.
Warjacks are big part of the game. Like I said before, your warcaster has rules for controlling Warjacks. They can also be controlled by jack marshalls, less powerful models who can also control them. Every Warcaster has a limited amount of Focus, which is used for controlling Warjacks and casting spells. So if you plan to field many Warjacks, it's a good idea to bring some jack marshalls to lighten your Warcasters burden.
If you don't like steampunk theme and playing with huge steampowered robots led by a powerful hero this is not a game for you. However, if you find that idea totally rocking, this is perfect for you!
There is one more difference that has caught my eye. There are LOT more special rules, weapons and spells for models in Warmachine than in 40k. This means that your models are really going to feel unique and there is great difference between units. This also means that it's going to take a longer time to really learn to use your models. It also means that sometimes it's hard to remember all of your opponents special rules, especially when playing against certain army for a first time. Luckily the cards make checking units a lot faster than going through a book.
The way your turn flows is different. In 40k, you have phases in which every unit will do one part of their turn. In the shooting phase, everyone will shoot and then you move in to a assault phase, where everyone will melee etc. In Warmachine, there is a maintenance phase in which you allocate focus to Warjacks and other smaller stuff happens. Then in activation phase, you will activate your units/models one by one. Every model will do their WHOLE activation before you move on the to next one. So for example, you can charge a model with a warjack and destroy him, opening line of sight for other units who can then shoot his Warcaster. The way this game works not only makes huge amount of combos and synergies possible, it makes them necessary for a win.
This is all I had to say for tonight, if you think there is another major difference I did not talk about, drop a comment below!