The Romans are considered a great civilization because their monuments and armies look cool. That is why they are so prevalant in history books, programmes and films. Coupled with the fact that there are lots of Roman structures still around it tends to make them easy for Europeans to identify with - it's easier to identify with something if you can reach out and touch it.
On the other hand, if you asked me why they
were a great civilization I would re-hash a great deal of what various people have already said in this thread. What they shouldn't really be credited with in invention of concrete and aqueducts - like so many things the Romans 'inherited' those inventions. Concrete pre-dates the foundation of Roma by thousands of years, and aqueducts were used by the Harrapans, Egyptians, Assyrians, Persians and (I think) Etruscans. The unique thing about Roman concrete was it's composition - I believe that it was the first concrete devised that would set underwater. This was almost certainly a fortunate coincidence - the Romans had to use the materials that were available to them in Italy to make their concrete, and it just so happened that the materials available made a concrete that would set underwater. Robert Harris did a very good novel called Pompeii if anyone wants to read something on Roman aqueducts.
One interesting thought - Romans built many of their monuments in concrete, which has very little value if you pull it down and try to re-use it. The Greeks tended to use marble blocks, which were very valuable to anyone who could carry it away. Clever Romans!
They may also have been one of the earliest civs to use wide, well built and well maintained straight roads, but they were by no means the first. Look up the Persian Royal Highway (2500 KM, built sometime before Alexander, therefore at least 350 BCE) or the Mauryan Grand Trunk Road (Taxila - Pataliputra, I'd estimate at least 1500 KM, built sometime before 200 BCE) if you want confirmation of that. The Greeks were in touch with the Persians and (through Alexander's successors) the Mauryans, so to say they would never have developed a widespread system of road building is an underestimation IMO.
The things I think of when I think of Roman civilization (both identified by Nanocyborgasm) are the pretty masterly administration they extended accross Europe, and their military doctrine coupled with incredible staying power that allowed Rome to win war after war.
In the end I think that the Romans were just very good at making the most of what other people developed, which, truely, is one of the greatest skills you could wish for. Mirc's post makes excellent reading I think, though I'd again disagree with some of what he says on the invention of concrete and Roman engineering prowess.
Finally, @ scy12:
You can't say they did not do on their own much .
Rome or more precisely the Roman civilization is part of the Greek civilization and the Greek civilization is part of Rome.
As other things great about Rome that the Greek civilization before encountering them lacked , was Roads , Bridges , use of Cement and generally Engineering. I doubt the Greeks would reach that level on their own on that areas.
Now read up on Archimedes, and
look at this, and tell me again that the Greeks were no good at engineering...

I'll not argue with the rest of what you said, but your statement on engineering seems to show an element of ignorance.
[General disclaimer] - Yes, I tend to be biased towards Greeks because I've read so much about them, but I've read a huge amount on Rome too to make up for it. As to the general cultural bias towards western civs: well, I'm western, so it's almost inevitable. I don't discount the achievements of other civs, but as an English speaker it can be hard to locate good sources of information on other ancient cultures. My opportunities are limited to an occational trip to the British Museum to view one of their occasional Persia or (upcoming) Babylon exhibits. I would be MOST grateful if someone could get me really high quality information in English on the make-up of the Persian Achaemenid Empire - how it was organised, military make-up, cultural and religious practices. Such things are most interesting to me, but frustratingly difficult to find any detail on.