- The Nix expression search path (`NIX_PATH`) no longer contains `/etc/nixos/nixpkgs` by default. You can override `NIX_PATH` by setting `nix.nixPath`.

- Python 2.6 has been marked as broken (as it no longer receives security updates from upstream).

- Any use of module arguments such as `pkgs` to access library functions, or to define `imports` attributes will now lead to an infinite loop at the time of the evaluation.

  In case of an infinite loop, use the `--show-trace` command line argument and read the line just above the error message.

  ```ShellSession
  $ nixos-rebuild build --show-trace
  …
  while evaluating the module argument `pkgs' in "/etc/nixos/my-module.nix":
  infinite recursion encountered
  ```

  Any use of `pkgs.lib`, should be replaced by `lib`, after adding it as argument of the module. The following module

  ```nix
  { config, pkgs, ... }:

  with pkgs.lib;

  {
    options = {
      foo = mkOption {
        # …
      };
    };
    config = mkIf config.foo {
      # …
    };
  }
  ```

  should be modified to look like:

  ```nix
  {
    config,
    pkgs,
    lib,
    ...
  }:

  with lib;

  {
    options = {
      foo = mkOption {
        # option declaration
      };
    };
    config = mkIf config.foo {
      # option definition
    };
  }
  ```

  When `pkgs` is used to download other projects to import their modules, and only in such cases, it should be replaced by `(import <nixpkgs> {})`. The following module

  ```nix
  { config, pkgs, ... }:

  let
    myProject = pkgs.fetchurl {
      src = url;
      sha256 = hash;
    };

  in
  {
    imports = [ "${myProject}/module.nix" ];
  }
  ```

  should be modified to look like:

  ```nix
  { config, pkgs, ... }:

  let
    myProject = (import <nixpkgs> { }).fetchurl {
      src = url;
      sha256 = hash;
    };

  in
  {
    imports = [ "${myProject}/module.nix" ];
  }
  ```

Other notable improvements:

- The nixos and nixpkgs channels were unified, so one _can_ use `nix-env -iA nixos.bash` instead of `nix-env -iA nixos.pkgs.bash`. See [the commit](https://github.com/NixOS/nixpkgs/commit/2cd7c1f198) for details.

- Users running an SSH server who worry about the quality of their `/etc/ssh/moduli` file with respect to the [vulnerabilities discovered in the Diffie-Hellman key exchange](https://stribika.github.io/2015/01/04/secure-secure-shell.html) can now replace OpenSSH's default version with one they generated themselves using the new `services.openssh.moduliFile` option.

- A newly packaged TeX Live 2015 is provided in `pkgs.texlive`, split into 6500 nix packages. For basic user documentation see [the source](https://github.com/NixOS/nixpkgs/blob/release-15.09/pkgs/tools/typesetting/tex/texlive/default.nix#L1). Beware of [an issue](https://github.com/NixOS/nixpkgs/issues/9757) when installing a too large package set. The plan is to deprecate and maybe delete the original TeX packages until the next release.

- `buildEnv.env` on all Python interpreters is now available for nix-shell interoperability.