1.  Servlet Dependency:
Actions in Struts1 have dependencies on the servlet API 
since the HttpServletRequest and HttpServletResponse 
objects are passed to the execute method when an Action is 
invoked but in case of Struts 2, Actions are not container 
dependent because they are made simple POJOs. In struts 2, 
the servlet contexts are represented as simple Maps which 
allows actions to be tested in isolation. Struts 2 Actions 
can access the original request and response, if required. 
However, other architectural elements reduce or eliminate 
the need to access the HttpServetRequest or 
HttpServletResponse directly.
2.  Action classes
Programming the abstract classes instead of interfaces is 
one of design issues of struts1 framework that has been 
resolved in the struts 2 framework. 
Struts1 Action classes needs to extend framework dependent 
abstract base class. But in case of Struts 2 Action class 
may or may not implement interfaces to enable optional and 
custom services. In case of Struts 2 , Actions are not 
container dependent because they are made simple POJOs. 
Struts 2 provides a base ActionSupport class to implement 
commonly used interfaces. Albeit, the Action interface is 
not required. Any POJO object with an execute signature can 
be used as an Struts 2 Action object.
3.  Validation

Struts1 and Struts 2 both supports the manual validation 
via a validate method.
Struts1 uses validate method on the ActionForm, or 
validates through an extension to the Commons Validator. 
However, Struts 2 supports manual validation via the 
validate method and the XWork Validation framework. The 
Xwork Validation Framework supports chaining validation 
into sub-properties using the validations defined for the 
properties class type and the validation context.
4.  Threading Model
In Struts1, Action resources must be thread-safe or 
synchronized. So Actions are singletons and thread-safe, 
there should only be one instance of a class to handle all 
requests for that Action. The singleton strategy places 
restrictions on what can be done with Struts1 Actions and 
requires extra care to develop. However in case of Struts 
2, Action objects are instantiated for each request, so 
there are no thread-safety issues. (In practice, servlet 
containers generate many throw-away objects per request, 
and one more object does not impose a performance penalty 
or impact garbage collection.)
5.  Testability
Testing Struts1 applications are a bit complex. A major 
hurdle to test Struts1 Actions is that the execute method 
because it exposes the Servlet API. A third-party 
extension, Struts TestCase, offers a set of mock object for 
Struts1. But the Struts 2 Actions can be tested by 
instantiating the Action, setting properties and invoking 
methods. Dependency Injection support also makes testing 
simpler. Actions in struts2 are simple POJOs and are 
framework independent,  hence testability is quite easy in 
struts2.
6.  Harvesting Input
Struts1 uses an ActionForm object to capture input. And all 
ActionForms needs to extend a framework dependent base 
class. JavaBeans cannot be used as ActionForms, so the 
developers have to create redundant classes to capture 
input. 
However Struts 2 uses Action properties (as input 
properties independent of underlying framework) that 
eliminates the need for a second input object, hence 
reduces redundancy. Additionally in struts2, Action 
properties can be accessed from the web page via the 
taglibs. Struts 2 also supports the ActionForm pattern, as 
well as POJO form objects and POJO Actions. Even rich 
object types, including business or domain objects, can be 
used as input/output objects. 
7.  Expression Language
Struts1 integrates with JSTL, so it uses the JSTL-EL. The 
struts1 EL has basic object graph traversal, but relatively 
weak collection and indexed property support. Struts 2 can 
also use JSTL, however it supports a more powerful and 
flexible expression language called "Object Graph Notation 
Language" (OGNL).
8.  Binding values into views
In the view section, Struts1 uses the standard JSP 
mechanism to bind objects (processed from the model 
section) into the page context to access. However Struts 2 
uses a "ValueStack" technology so that the taglibs can 
access values without coupling your view to the object type 
it is rendering. The ValueStack strategy allows the reuse 
of views across a range of types which may have the same 
property name but different property types.
9.  Type Conversion
Usually, Struts1 ActionForm properties are all Strings. 
Struts1 uses Commons-Beanutils for type conversion. These 
type converters are per-class and not configurable per 
instance. However Struts 2 uses OGNL for type conversion. 
The framework includes converters for basic and common 
object types and primitives.
10.  Control Of Action Execution
Struts1 supports separate Request Processor (lifecycles) 
for each module, but all the Actions in a module must share 
the same lifecycle. However Struts 2 supports creating 
different lifecycles on a per Action basis via Interceptor 
Stacks. Custom stacks can be created and used with 
different Actions as needed.