That's a complicated want list with things that are fundamentally in conflict. Here are what I think are the key thigns you're asking for:
landscapes and people (wide angle zoom)
flowers and occasional macro-style shots
birds and critters (big, powerful telephoto)
Body $2000, lens $2000 (max, $1500 preferred). So, $3500 total.
First suggestion -- buy a less expensive body. They're very capable, they weigh less (my T2i is much lighter than my 7d) and it'll do you well until you grow into and can afford a more expensive/capable camera. The thing to remember on camera gear is that you'll upgrade/replace camera bodies a lot more often than you'll replace lenses, if you buy quality lenses, so under-spend on the body, and put your budget into better quality lenses. it's a better long-term investment.
Also, you're right to not be impressed with the kit lens. But it is decent and can get you started. Or consider going to a third party lens like Tamron or Sigma. options:
T4i + Canon 18-135 $1100 (Amazon).
T3i + Canon 18-135 $850 (Amazon).
T4i + Tamron 18-200 ($800+$300 = $1100) (Amazon)
T4i + Sigma 18-250 ($800 + $425 = $1225) (Amazon).
For the price of the kit lens, you can get a similar (but better) lens from a third party. It'll give you a bit more reach and be sharper. I've tested the Sigma, and like most superzooms, it softens up at the telephoto end, but it'll still be a very usable lens that'll handle your needs for landscape and street/carry around type work. Sigma lenses are, in general, better built than Tamron lenses, but Tamron is a good lens set as well, but won't hold up to banging around as much. I've owned both types of lenses (still have my Sigma macro) and they're fine for what they are, especially starting out, but you will likely outgrow them as you advance as a photographer.
I put the t3i in there. As a one-generation-older body, it can be a real budget saver, but the advances in the t4i are enough that I suggest you buy the new body. But it is an option. the t3i would work fine with either of the lenses I suggest.
Then, bird photography and critters. For this you need some serious magnification. My go-to lens is the Canon 300 F4 plus a 1.4 teleconverter. I used to use the 100-400, but I ended up switching. For this, zoom is somewhat underrated, in practice, I used the 100-400 at 400mm about 95% of the time. If you use the sigma superzoom (18-200), a 300mm and a 1.4x, you get good coverage throughout the range to 420mmF5.6. That's why I prefer that lens combo compared to the 400mmF5.6.
Canon 300F4+1.4 ($1300 + $450 = $1750)
Canon 400F5.6 ($1300 -- no IS)
canon 100-400 ($1500)
Sigma 150-500 ($1000)
The Sigma: lots of people use it as a budget lens. Lots of people have said it gets very soft at 500mm. Some photographers consider it unusuable at 500mm. Others like it. I'd suggest borrowing/renting a copy and testing it first. I think there are better options for not much more money.
Canon 100-400. This was my primary lens for years. I recently my thoughts on it here: http://bit.ly/PZy7kA -- I think it's a good lens but older technology, and it's a lens I think can only handle a moderate amount of banging around. It's one lens I'd be very hesitant to buy used, and older units I think seem to vary in absolute sharpness more than newer units.
300+1.4 vs 400. 400 is a bit sharper. 300+1.4 is a bit more flexible, has IS, and I found it's AF was faster, even with the tele attached. A bit more money, but worth it. And I lean towards IS whenever practical because it can be a an image saver in the field in failing light while handholding. The 400mm is a stop faster, but you'll get fewer acceptable images handholding.
What you won't get with any of these is "lightweight". They're all big, heavy pieces of glass. all weigh roughly the same. all are fairly bulky. but if you want to shoot birds and critters, you need something like this. Nothing cheaper/lighter/smaller will make you happy for long or give you many pictures you like ("see that tiny blob in the distance? that's a grizzly bear!") This is where you start investing for the long term.
My suggested kit:
Sigma 18-250 ($425)
Canon 300 F4 ($1300)
Canon 1.4x ($425)
That's about $3000, or $500 under your lower max. Enough for batteries, memory cards and etc. This is a kit that can be handheld, but you'll want a tripod for landscapes. Lots of topics here on that.
If you prefer, go with the 100-400. I can't argue with buying that one, if you buy it new. If you do, then you can change your wide angle to Sigma 17-70 ($450), which is a higher quality lens.
Sigma 17-70 ($450)
Canon 100-400 ($1600)
That's $2850 or so. (all prices today @ amazon)
Either kit's a great kit, especially getting started. And you're investing in the high quality telephoto that you'll keep around for a long time, and the rest you won't feel terrible about upgrading from when it's time because you kept the costs reasonable. None of it you'll outgrow quickly, but be aware, with birds, you'll be lusting for 500mm and beyond fairly quickly. And that's not cheap.
Don't forget to consider the used market. And don't forget you can sell back gear when you want to upgrade. The tamron/sigma lenses typically have much weaker resale prices in used than Canon labeled, but it's there. I'm now buying more of my lens upgrades used. One nice option here is when the rental houses (borrowlenses or lensrentals.com) sell off inventory, and I also monitor keh.com and adorama and b&h's used inventory. You can save some money there. If you go the ebay/craigslist route, be careful about fraud or inflated quality ratings. I'm willing to pay a place like keh because their ratings are trustworthy.
Good luck, and have fun!