I am currently using an entry level DSLR (Canon 550D). In the future I am looking to move from my current camera to something more professional. My use would be general use including macro, wildlife, sports and maybe a bit of portraiture.
My questions are:
Before looking at the next camera, make sure you have the best lenses for your needs. A good macro lens for example will make much more difference than a change in camera. The same is true of long fast lenses.
If you have enough good lenses though, it gets hard to change camera brands, so the first factor to consider is if you are willing to change brands should there be something more suitable by another manufacturer. Sticking with the same brand, makes the choice easier :)
Higher-end models are primarily made to let you work faster. You mentioned sports and wildlife which are excellent use cases for this and this says that you should consider faster frame-rates as of higher importance than higher-resolution. Canon knows this too and has made their faster model (EOS 1D X) much pricier than the highest resolution one (EOS 5D Mark III). Nikon did the same with the D4 (fastest) and D800E (highest resolution).
Portraiture and macro are less demanding but if you do extreme macro, Canon is the way to go, they have the only DSLR-compatible 5X magnification lens, the MP-E 65mm F/2.8. Getting the same magnification with any other brand is quite inconvenient.
For studio portraits, depending on your lighting equipment you can look for something with a sync-port. You will notice a Sync-Port appears on the EOS 7D and higher end models in the Canon lineup. The more modern way of shooting with studio lighting is to use wireless flash triggers which work using a standard hot-shoe rather than sync-port.
The 7D is also well suited for action with a frame rate of 8 FPS. Not only that, as a cropped-sensor camera, your lenses get more reach which is a great bonus for wildlife and certain sports.
The closest equivalent from other brands are the Nikon D7000 and Pentax K-5. Nikon, just like Canon, lets you grow further from there with full-frame models higher in the lineup. They also have an extensive line-up of lenses where Pentax much fewer and is missing long telephoto lenses which gets them complaints from wildlife photographers. Sigma fills the gap somewhat.
Unless you have a limitation in mind with the body you have, whether that be autofocus speed, autofocus accuracy, low-light ISO performance, fps - then you should probably invest in lenses, flashes/triggers, tripod and/or software.
Lenses make a much bigger difference in sharpness, contrast, clarity than upgrading your body will.
Lighting is second in how much difference it can make in your images - so think about picking up a speedlight and some wireless triggers. Some cactus triggers and a Yongnuo flash will get you rocking for ~$100 and will make a WORLD of difference.
You should have a need in mind when looking for a new camera instead of just desiring a new camera. If your current camera fits your needs, keep it and put the money towards new lenses or something.
For example, I recently purchased a newer (more pro-ish camera replacing my 1000d with the 60d). I wanted it because it has a much better autofocus, has the ability to wirelessly trigger my 430ex ii, and had some custom functions I was interested in.
So what features/specs do you need? Do you need more FPS because you're shooting sports, less noise at higher ISO because you're shooting in low light a lot, etc? These are questions you should be able to ask yourself once you take a lot of pictures and find how your current camera isn't meeting your needs.
I think basically now is the time to look hard at lens lineups and how they are different. But I think you're essentially in this situation: After 2 years of amateur photo, buy a new body or a great lens?, but with more freedom since you don't have a large lens investment.
If you're really hating the ergonomics of your current camera, or tempted by something on the market (maybe a switch to a mirrorless system), switching brands is worth investigating. But on the whole, the Canon 550D is a fine camera, and you are probably best off sticking with it for a little longer and investing that money in nicer lenses, before upgrading the body.
First, if you didn't use that entry-level DSLR for at least a year and didn't read 3-5 books on photography - I wouldn't even think about buying something new. The idea that a new camera or lens will greatly improve your photography is usually false.
Otherwise, as others already mentioned, determine how your current camera limits your creativity, and try to solve it.
But before that I'd suggest doing one thing that can prevent you from spending lots of money. Buy an ebook by a great photographer. Pick anyone - David DuChemin, Jay Patel, someone else. It will not cost you much. Try to do everything that's written there without judging. Just go page by page and do it. Chances are your photography will improve greatly and an answer to your question about equipment change will come much more easily.
This can be hard even for some professionals deciding when and what to upgrade. As tenmiles already said it is mostly down to individuals and what kind of photo they want to shoot. Sometime people asked me what camera they should buy but when I asked what kind of photography they do, I usually get the answer as "everything".
When I want to make an upgrade I usually look for,
built in auto focus motor - this can be important sometime, if you have older lens or cheap lens as 50mm f1.8
compatibility with accessories - sometime you could have invested more money on accessories more than your camera body. These often include wireless trigger and flash units which mostly work for only one brand.
compatibility with the existing lens collection - most people stick the brand they started with as you could spend collecting lens. If you have a good lens, it will stay with you for decades but not a camera body.
Full Frame or Crop Frame. Most camera manufacturers produce full frame and crop frame cameras. In my experience, the image quality differ hugely as the price differ hugely as well. One thing to keep in mind is Crop Frame lens (DX lens on Nikon and EF-S lens on Canon) are only compatible with Crop Frame body. You can still fit on crop mode on Nikon but what's the point.
And other factors such as ISO, Bracketing, Frame per second are also important. Personally, as a landscape photographer, I never care of auto focus, frame per second or ISO. Bracketing is a good plus for me though.
Throughout my photography journey, I often wanted to upgrade my camera body to improve my photography but this is sometime wrong. You can keep with your camera body and may think to buy more accessories, such as Filters for landscape, flashes or softbox-es for Portriats, etc etc. Upgrading the lens usually produce better photo for me than a better camera body with a crap lens.
For example, if you have Canon 550D, you won't want to upgrade to 600D or 650D. Those may be better camera than yours but it won't make much difference. Personally, if I have 550D, I won't make an upgrade unless I upgrade to 5DM2 or at least 7D.
Hope that helps.
PS. I am a Canon fan. I won't want to change to other brands unless I become a millionaire, I will buy a Leica Sx.
I would say the obvious main deciding factor is budget, second to usage requirements.
The more you spend on a camera the LESS it will help you (IE they start to remove shooting modes) - however the size, weight and build quality go up, along with features such as Wireless iTTL flash triggering, lens motor drive and FPS.
I would say, first step, decide how much you want to spend, the see what fits in your budget.
As for deciding on brand... if you are used to a Canon, I'd stick with them, plus that means you only need a body and can use your current lenses. As for future-proofing, buy the newest model you can, or even wait for a new release, i tend to be on a 3 year cycle at the moment, which suits Nikon's release rate.
I'm pretty sure that your #3, "How should I decide between brands? I do not have a large investment in lenses." has been covered a bunch of times.
Fundamentally, Canon and Nikon have nearly all of the serious DSLR market, with Sony and others a much smaller part. The main thing that this causes is that Canon and Nikon have huge selections of lenses and accessories. This essentially locks you into one or the other brand. Once you have a bunch of lenses and accessories, it becomes too expensive to change.
Between the two, pick the one you like. Like the looks? or feel? or controls? get that one. Do not compare features. In six months, whatever cool thing one brand has will be available in the other. They compete, and they compete seriously.
Remember, bodies are simple containers that hold a computer. All computers become obsolete quickly. Lenses last decades if well cared for.
You should list your priorities. I'd say for macro or portraiture any cent spent on the body is wasted since you have not spent it on a better lense. Even if you say, you have not made a big investment into the lenses, I would be very careful to change the manufacturer, because you will not only have to buy new lenses, but have to change to a new user interface. I'd even go so far as to say that you will not gain from switching the manufacturer, since the features of the semi-pros from every manufacturer will be so close that it doesn't make a difference for the average amateur.
