There are no unlocks in Waves. None at all.
If you buy the game, then you get access to all five game modes right away. This won't prevent most of you trying them all out in the order they appear in the menu, but there is absolutely nothing stopping you from starting out with Bombing Run rather than Crunch Time.
This is for a reason and that reason is not "Too lazy to put in unlockable content".
The reason there are no unlocks is because the moment you have unlockable content gamers start to focus purely on unlocking the content rather than enjoying the game itself (See The Overjustification Effect).
Watch this. The relevant bit is at around 3 minutes in.
I'm going to use an anecdote to support my position on unlocks (even though there is plenty of empirical evidence that supports this theory anecdotes are fun and more relate-able)…
I recently started playing Space Marine multiplayer. It's a lot of fun, when it isn't lagging (which is sadly most of the time), but it has a few too many unlocks. Ever since I started playing, I have been playing as a pure Tactical Marine—Bolter and Frag grenades only. At first this was because I just love the Bolter (it's an awesome feeling weapon to use) but, after a few hours, it was because I wanted to unlock the Kraken Bolts for it. Then once I had unlocked the Kraken Bolts I wanted to unlock the Targetter (Which I still haven't it being bloody hard to do). I was refusing to let myself play with any other class or weapon combination, because I wanted to unlock one of the rewards (which I suspect once I had done I would use for two games and then try to unlock something else).
My quest to unlock everything for my Bolter was starting to affect how I enjoyed the game. It was no longer enough that I scored a kill or if my side won the game—if I hadn't gotten at least one Bolter Kill Streak out of the game then in my eyes it was a failure. I was completely single-mindedly focussed on unlocking content to the point where I had stopped enjoying the game and was playing in a way that was completely selfish and detrimental to my teams overall effectiveness (if it wasn't for the fact that everybody else was playing this way, I'd feel a bit more guilty about it). I'm now making myself play with whatever I want to or feel the situation demands rather than trying to unlock specific items one at a time. I may unlock the content eventually but I will do so in a much more organic manner over a longer period of time. Above all I'm having fun again.
Now I know that one of the reasons for having to unlock content is to ensure that the player has learnt how to use the pre-requisites sufficiently–-but in the majority of cases the unlocks exist to keep people playing a game they may not be enjoying because they have sunk a lot of time into it and would feel horrible if they just abandoned it. Unlockables are more often completely cynical methods of trying to extend playtime (and, coming from AAA console development, I have been in the meetings where the solution to a game being 4 hours too short is to add some hidden objects that unlock concept art so I know it happens). In some instances the addition of unlockable content can completely undermine the entertainment to be gained from just playing the game.
Waves is about enjoying the game itself. It rewards good players with increasing difficulty rather than decreasing (which is often what unlocking powerups achieves) – the better you get at the game the harder the game will be in response. It's designed to keep you in a sweet spot where your abilities are being pushed just enough to keep you from being bored but not so much as to make you frustrated (See Flow) – the score is only there so you have feedback on how well you're doing (and also so you can gloat to your friends).
Waves can never be completed but it can be exhausted. It is not possible to "100%" the game but it is possible to reach a point where you feel that either you aren't going to get any better at it or that you would rather spend the time doing something else. I don't mind if you get bored of Waves and stop playing as it is in fact inevitable that you will. I just hope that while you are playing it that you're enjoying it for the experience of playing, learning and improving rather than because you persevered long enough to unlock everything in it.