There are a lot of good common sense replies to your query already, however I haven't seen (maybe I missed it) probably one of the most important reasons to pick any brand - the lenses and your use for them.
You need to consider what you are going to be shooting & what your budget is and maybe let us know.
If you are a sports shooter then you need fast AF and there Canon / Nikon rule the roost (not that other brands can't shoot sports and do it superbly too) and low light is becoming less of an issue nowadays as cameras can handle higher and higher ISO's which means less expensive lenses can also handle low light (e.g. at least an F2.8 300 was de riguor but now with high ISO capability an F4 300 - which is much less expensive - can easily handle the same shot).
For portraiture, street, landscape etc. then many other systems can offer you just as many options so don't rule out Sony, Pentax, Olympus etc.
For macro it's the lenses more than the camera that are important.
Maybe you need a light kit, something to take with you everywhere - in which case the micro 4/3 Olympus or Sony Nex are near perfect.
If you need full weather resistance (for shooting in inclement weather, in outdoor sports or on trips to challenging locations - arctic, jungle, windy and dusty / sandy environments) then take a very close look at Pentax for their superbly weather-proofed, but not expensive, bullet-proofed DSLRs. The Canon 7D would be another option but full WR lenses are Pentaxes speciality.
If you are on a budget but still want great glass then Pentax make wonderful lenses and virtually any lense they have ever made over the last fifty years will sit happily on their very latest cameras (although their fantastic primes cost a lot - not compared to Canon L glass which are far more expensive) and gain instant IS/VR/SR since Pentax (and Olympus) have the shake reduction built into their bodies - it may be better in-lense but those lenses are very expensive to gain only maybe 1 stop over on in-body option.
Choose your camera by selecting your lenses first - by focal length, to suit your budget and by quality - and you won't go far wrong, cameras come and go but the lenses keep on going (and don't lose their value unlike cameras) ! Note that all of the good manufacturers (Canon, Nikon, Pentax, Sony & Olympus) have some wonderful lenses and probably more than enough to suit your requirements now and in the future (unless you require long long reach then Canon / Nikon do that best - but in exceptionally expensive lenses costing in the thousands of dollars). There are also some fantastic lenses to suit all mounts from Tamron / Sigma & Tokina to name just three. Lenses that will in many cases out-perform even the camera brand's own more expensive lenses - at a much lower prince point.
Also consider the crop factor (if wanting to shoot wide or have long reach is a major consideration) :
Canon x1.6, Nikon, Sony & Pentax x1.5 and Olympus x2 (great for reach not so good for wide).
Some high quality & top quality (all APS-C not FF) cameras to consider :
Canon 7D & 60D, Nikon D7000 or 300s, Sony A55, Pentax K5 or Kr (both of these have the superb new Sony sensor - as does the Kx but without the latest tweaking).
Probably the best value cheaper DSLR and kit lense set on the market is the Pentax Kx with 18-50 & 55-300. Roughly US$500- 600 if you want to save money and still get a great performing, low light, camera.
Finally I would not choose your DSLR just because your friends have the same brand. There is more than enough information on the web to tell you all you need to know, and answer all your questions, on the brand you choose. Every brand has excellent forum(s) where you can learn from experts and have your queries answered. Do your research and get your hands on the cameras first !
