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Cladistics has pretty much debunked the idea of a Class Dinosauria.  No more
artificial Linnaean rankings -- now all of the groups are clades, set in a
hierarchy but without the pigeonholing that Linnaean taxonomy demands.
According to cladistics, birds are nesting high in the tree of Dinosauria, so
they ARE dinosaurs.  (This point cannot be stated enough.)  Dinosaurs are NOT
birds, but the vise versa is true.  Classifying some "traditional" dinosaurs
(those that are not birds) AS birds is not so ridiculous as long as they have
all the features that mark a bird.  It would depend upon how Avialae (or
whatever the basal bird clade is) is diagnosed.  

Of course there are common features between "dinosaurs" and birds!  You are
expected to resemble your ancestors.  The differences between, say, a
Velociraptor and a pigeon are to be expected, since evolution = change, but
just because an organism changes doesn't mean it becomes a separate biological
entity.  Triceratops looks a heck of a lot different than Lesothosaurus, but
that doesn't mean Triceratops isn't an ornithischian anymore.  Likewise, just
because a pigeon is very different from Eoraptor, that doesn't mean it is no
longer a theropod.

In addition, dinosaurs ARE reptiles.  Reptilia was redefined recently (it's no
longer a paraphyletic clade) as one of the main offshoots of Amniota (the
other being ?Synapsida?).  So, birds, strange as it may seem, are reptiles,

Dinosaurs don't belong in their own "Class."  They're just another clade on
the great tree of life.  

Rachel Clark

"What's up, homies?"