The WFRP2 system is close to WFRP1, but better balanced. It still uses a percentile D100 as the basis, character's characteristics are pretty much the same, the career system is still present, etc.
Major improvements or evolutions are on:
- The magic system is completely new, runs properly, and does not require as much xp as was the case in WFRP1.
- A list of basic skills is defined, based on characteristics scores - Skills now give a bonus when attempting to use that skills action.
- Special abilities or feats are no longer Skills but are now Talents.
- Careers are better balanced (in WFRP2 most of the basic careers offer the same amount of skills, characteristics enhancements, etc. - Unlike the wild variance in starting careers in WFRP1)
- There are many optionnal tactical rules for fights (special abilities for weapons, different postures for attacks: quick attack, brutal, defensive, etc.)
- A few characteristics have been merged (for instance WP in WFRP2 covers Cl and WP from WFRP1)
These are the major technical differences I see, there are probably others.
At the background level, WFRP2 takes place a few years later than WFRP1, after the chaos incursion led by Archaon. The Empire has been partially laid to waste and is under reconstruction.
The background closely follows the Warhammer Battle background from when WFRP2 was released, meaning there are differences from the WFRP1 background:
- Magic is divided in 8 colors (fire, light, death...)
- Some gods are no longer part of the background (Malal, Arianka and globally all the Loyal gods...), Chaos gods are the 4 one well-known (Tzeentch, Khorne, Slaanesh, Nurgle - rememeber WFRP1 core book did not even mention Tzeentch), Sigmar is now a 'playable' religion for priest characters (in the same way, Sigmar got barely a mention in the core WFRP1 book)
- Some races and creatures are not part of the background anymore (fimirs, slanns, zoats...)
- Background (The Empire, cities...) are much more consistent and coherent in WFRP2
- There is an "undead-vampire" region (Sylvania) in the Old World of the Empire
At the player level, you can access a few new and exotic careers (flagellant, Estalian diestro, Kislevite kossar...)
The line of products has been quite well managed with a good balance between scenarios and background information. There are books for the major themes (The Empire, Chaos, Religions...). A few books explore other regions or countries (Bretonnian, The Skaven, etc.). There are three new campaigns, but none achieve the same level of reputation as The Enemy Within did for WFRP1.
WFRP2 books are nice looking (the majority are printed in color with lavish illustrations) and contain a lot of ambiance text.
The Corebook of WFRP2 is not quite as complete in comparison with the WFRP1 corebook: a bestiary in the core rule book is incomplete (it is instead a dedicated supplement), the background overview is not nearly as detailed... However, the WFRP2 corebook contains all that is needed to play.
If you know WFRP1, WFRP2 is not mandatory, but will give you 'fresh' material to use with both WFRP1 or WFRP2, and offers a more balanced system.