They're also spiky, heavily armoured and usually filled with heavily armoured spiky things... and I LOVE EM!
Given the opportunity I'd quite happily field a Land Raider for all three of my heavy slots. 30 Berzekers wrapped in a blanket of steel and lascannons, not too shabby... through in a havoc launcher and a dozer blade to stop em getting stuck on rocks n stuff (and cos they look cool) and you're in business.
Unlike those silly Imperial types, us Chaos chosen don't have access to a billion different types of 'Raider, so we have to get creative. Most of our war engines date back to the Great Crusade itself, and this allows (as with most things Chaotic) A LOT of scope in terms of customisation and conversion work.
I love how Stempe over at Brush And Miniature Torture has retrofitted his Iron Warriors 'Raider with "old school" side sponsons (scratch built and bang on the money), and Hortworth's Khornate offering is simply jaw dropping!
I myself am still plodding away with my own dozer clad war machine, and hope to have it finished in the coming weeks (preferably before the new baby arrives and takes the last of my free time from me).
What I really wanted to say in this post, is don't be limited by just what's in the box. Even for those Imperialists out there, there's still plenty of inspiration like the ones found in this Bolter and Chainsword thread. (I especially love the mini dozer blades, a rather elegant solution to the "assault ramp" problem)
So don't be content with just opening the box and gluing it together, whilst this can be great, there are greater things out there if you're willing to put in a little build time, and plenty of tutorials and simple conversions for those who aren't as accustomed to chopping up plasticard and fiddling with green stuff as others.
Remember, inspiration is just a google away.