Firstly, you don't need to be a huge comics fan or know anything about the 'Guardians of the Galaxy' going in to this to enjoy it. Even though I'm a pretty big Marvel fan, I still knew very little about the Guardians, but had heaps of fun with the film anyway. It's a pretty simple concept - a boy, Peter Quill (Pratt) is taken from Earth by one of the spacefaring races that exist in the Marvel Universe, grows up as a Han Solo-esque outlaw calling himself 'Star Lord' travelling/thieving around the galaxy. He steals the wrong thing (an orb) and gets involved in a galactic power struggle which forces him to work together with Gamora (Saldana), Rocket (Cooper), Groot (Vin Diesel in his best role ever!) and Drax (Bautista). Action and adventure ensue!
The bad guy, Ronan (Pace), is a Kree that wants to wipe out the plant Xandar - no real reason why he hates the Xandarians, he just does! - and Thanos (who we saw at the end of 'The Avengers') is going to help him, once he gets the orb. It sounds complicated - and we haven't seen any Skrulls or Shi'ar yet! - but the film doesn't really get too bogged down in galactic politics or grudges. However, it sets up the potential for a huge 'The Avengers 3' or the new 'Fantastic Four'.
One of the best bits is that Quill was abducted in 1988, so the only cassette he has is full of 70's & 80's music, meaning he knows nothing else and the film does a great job of throwing in catchy, but not necessarily huge, pop songs throughout - Bowie, 10cc, Blue Swede, Joan Jett, Marvin Gaye... The audio makes everything fun and provides a nice contrast to the aliens/space battles going on in the visuals!
So, when Quill and his team of misfits - so many 'Star Wars' comparisons (a good thing!), including Thanos appearing as a hologram to Ronan - decide to do something good with their lives and join forces with the Xandarian "Nova Corps" (although, no sign of Nova the character) to stop Ronan. There's a huge/exciting battle and final scene.
Plenty of laughs throughout, mostly thanks to Pratt & Cooper and the film never gets bogged down in too much seriousness, or the darkness/evil of the villains. Loads of eye-popping action and lots of cameos/support actors adding to the overall "big galaxy" feel. The Groot scene near the end is spectacular and the post end credits scene is a pretty great inside joke.