AbdulAlhazred
Legend
I think what [MENTION=996]Tony Vargas[/MENTION] is trying to say is that running an encounter of 3 level 2 monsters vs 3 level 2 PCs is what is considered an 'easy' encounter, unlike 5e CR where a single CR2 monster is supposed to take on a level 2 party. The DMG claims you can have up to 'level + 4' encounters (very hard), though experience has shown that this is not actually the upper limit. At low levels you DO probably want to keep it in the level -1 to level +3 range, at higher levels things can get more flexible (and of course this is just considering monsters alone and some fairly neutral terrain and no other major considerations).
I would say that 'MM3 grade' monsters are, on average, tougher than 'MM1/2 grade' monsters, while being more interesting to run and fight. Not to say that MM1 and MM2 (and other early books) lack good monsters, there's plenty of fun to be had. They just tend to be underpowered in damage output, sometimes have unfortunate designs vis-a-vis what conditions they dish out (IE stunned with a duration of 'save', not fun).
I would say that 'MM3 grade' monsters are, on average, tougher than 'MM1/2 grade' monsters, while being more interesting to run and fight. Not to say that MM1 and MM2 (and other early books) lack good monsters, there's plenty of fun to be had. They just tend to be underpowered in damage output, sometimes have unfortunate designs vis-a-vis what conditions they dish out (IE stunned with a duration of 'save', not fun).