Currently I run a campaign using Fate system, which means that there are no accidental deaths. This allows me to use a vary wide range of difficulties. Characters get defeated, knocked out, forced to run or surrender quite often and quite often they have overwhelming victories.
I'm running four games at the moment: 30th level default 4e; 1st level 4e Dark Sun; Marvel Heroic RP with some X-Men and War Machine; and Burning Wheel.
The high level 4e game lets me design encounters basically as crazy as I like - PCs at that level are virtually unkillable, and the stakes are "high cosmic". The 1st level game, by contrast, involves squishy PCs (a rogue, a monk, a bard, a barbarian - plus a (non-squishy) battlemind) and I haven't pushed too hard so far.
It's pretty hard to kill PCs in MHRP, though not impossible, and Nighcrawler currently has d10 Mental trauma (an attempt to heal it only made it worse). But so far the PCs have only lost one encounter (which I ended with a 2d12 Doom Pool spend when Titanium Man was winning against War Machine).
BW is the most "gritty" of the four games (by quite a margin). The system is set up so that the PCs lose quite often. The main PC, a wizard, was captured at the end of a session a couple of months ago and (at the start of the next session) regained consciousness inside an iron maiden. After breaking his way out and then reaching a mutual accommodation with the death cultists who had captured him, he tried to read a mummy's aura with the result that he was inflicted with mummy rot. He then was able to meet a holy man who healed him, but the cathedral then turned into a a bit of a bloodbath: the PC's demon-possessed brother arrived on the scene, inflicting a near-fatal wound on the abbot; the PC then locked the brother down with successful grappling; but then another PC, sworn to kill the brother, turned up on the scene and knocked out the first PC; in the conflict between that second PC and the brother, the PC rolled poorly while the brother rolled well, inflicting a near-fatal wound on the PC; the first PC then regained consciousness, in time to see the brother try to call down a mighty spell to destroy the cathedral only to fail and inflict a mortal wound on himself as the spell was internalised. The PC's attempt to save his brother from death failed; but the other two severe wounds (on the holy man and the second PC) were staunched, though each then required months for recovery.
Next session will begin three months later. (The rest of the PCs, who didn't need to heal, will get the benefit of 3 months practice.)
Removing character death from the table gives players freedom to take risks and express their characters even when it's tactically bad idea. It also gives me freedom to stat NPCs without much thinking about balance and to push really hard when it makes sense in the fiction.
All the systems I'm running allow the GM to push hard (in 4e, this is first by building to guidelines, which are pretty reliable, and then pushing hard). BW does reqire a bit of thought about balance, but not too badly. MHRP is pretty relaxed about balance, because the dice pool system is more like a lot of re-rolls, rather than stacking big numbers.
I like it when players can take risk and express PCs even when it's tactically bad. In 4e, this works because mechanical tactics don't always correlate to "in fiction" tactics (eg some PCs get bonuses for being surrounded by bad guys rather than trying defeat-in-detail). In MHRP, tactics aren't really part of the system. In BW, tactics are important but so is earning "artha" (fate points etc), and the latter are conditioned on playing the PC rather than doing the "sensible" thing.