Disclaimer:
I work for Stardock which makes Start10. That means I have a vested interest in the quality of the Windows 10 start menu. The better the start menu is, the fewer copies of Start10 we sell. On the other hand, the worse their start menu is, the fewer copies of everything else we make will sell. So overall, I would prefer that Microsoft make Windows as good as they can.
At least it’s there
Windows 8 didn’t have a start menu at all. It had a start screen. As a result, Stardock’s Start8 got tens of millions of downloads. This time, they have brought the start menu back…sort of.
The Good
I like Cortana as a concept. It’s a good idea. The implementation needs work but I look forward to one day being able to say “Computer, order my wife a dozen roses for tomorrow, put on the note, ‘I’m sorry about the chicken..incident.’”

Cortana is a great idea even if I have trouble spelling her name.
I also really like the expand start menu option. If you select it, you get a quasi-start screen. This is a great UX and one I wish had been in Windows 8.

There’s an expand button in the top right that lets you expand the start menu.
I also really like the Recently added item. This was a brilliant addition in my opinion. You install something, it shows up right in the Start menu.

The “Recently Added” option is great!
I also like the general design of Windows 10 better than Windows 8 or Windows 7 in the sense that Microsoft is making a better distinction between consumer users and power users. For example:

The Start menu right click menu is helpful in getting you to core management features. First introduced in Windows 8.1.
Take a close look at what they display here. If you’re a power user or someone with just a lot of experience, you know every one of these. There’s no need for these to be jammed into the Start menu as some pretty icon. These are the low level controls of the machine. Note that even the CLI is there.
The Bad
So there are definitely some good things in the new Windows 10 start menu. Even things that Windows 7 users could appreciate. But if this start menu were to go out as the final, there will be many millions of new Start10 customers. Here’s where things go bad.
Ditching the document centric purpose of a desktop
First, the search bar has zero usability. Let me walk you through just how terrible it currently is:

I have a document called “My Budget” . The point of a desktop PC is to manage my stuff. A lot of stuff. Otherwise, I’d just a tablet.
So I type in “My budget”. Now what. Now nothing. There is no context menu. Let me be clear: THERE IS NOW RIGHT CLICK MENU. That means I can’t even pin that item to my start menu or open its file location or choose to open it with a different file. I work with XML files all the time and what program I use to open it with changes.
I’m going to assume that Microsoft will fix this before release. But I point out this issue because I’ve seen people who are supposed to be technical, poo-poo Start10 even thought Start10 beta lets you type in this sort of thing and right-click to get that file type’s context menu, pin it to the Start menu, etc.
No jump lists

I use a desktop to do work. Not having jump lists is a problem.
Jump Lists are a big deal because they allow users to go directly to the most frequently used documents of a particular app right from the Start menu. So if I’m using Word, that contract I was working on is visible right away, for Visual Studio, the 2 projects I’m coding on are readily accessible. That’s simply gone here.
The left hand and right hand don’t know what they’re doing
The left side and the right side have nothing to do with one another. You can’t drag and drop things to the right or the left.

The left side and right side might as well be separate programs
This is actually two problems in one. First, I would expect to type into the search bar (which I’d prefer to be in the Start menu itself but I digress) and when it brought up the document or program I’d expect to be able to pin it to either the Start menu or “add to places”. Second, I would expect to easily drag and drop things from Places to the tile list based on what I am working on a lot that week.
Speaking of Places…
First off, you have no say in what shows up in places. I bet a lot of users assumed that places were the same as the pinned items in Windows 7. They’re not. They’re currently hard coded.

A minor semantics quibble
Right now, you can’t add or remove things to “Places”. I would prefer to see it named “Favorites”. A user could then pick and choose what shows up in Favorites. Not to mention, File Explorer and Documents are essentially the same thing. One just opens Explorer to a particular location.
All Apps
The All apps option is one of the most frustrating things about the new Start menu.

Luckily users rarely have anything that starts with an Mi.. on their system…
First, there’s no way to organize this. You can’t simply create a “Stuff” folder and place all the things you rarely use in it so that when you do need to view apps that you don’t have on your start menu, there’s some semblance of organization. In fact, there’s no ability to organize it at all.
For example, let’s say you’re one of the millions of people who don’t know about the right-click on the Start menu trick. For a long time you’ve been trained that if you want to remove something on your computer you go to add remove programs. On this Start menu, too bad. The search won’t find it and it’s not in the “all programs” list.
The Elephant in the room
Live tiles on the desktop are a bad idea. Nobody knows this concept better than I do. Stardock, makers of ObjectDock, DesktopX, ObjectBar, Tiles, ControlCenter, Desktop Gadgets, etc. know something about this concept having been doing it for going on 15 years. Weather temperature, stock prices, calendar info, news headlines and email notifications are what you’re going to get. It’s what you always get.

Can someone really make the case that this is a good UX for a mouse using desktop user?
For 95% of the Windows 10 installed base, live tiles are, in effect, poorly made, gigantic icons. If we thought that a wall of ugly icons was a good idea for UI, we would have had them years ago. The only purpose for tiles is to make touch computing easier. Of course, I would also point out that 2 color tiles are a bad UX as well. Humans are great at recognizing patterns. The more distinction you can make from one tile to the next the better. Instead, they all look roughly the same.
Bottom line
While the Windows 10 start menu is an improvement over Windows 8, it is still a substantial step back from what has previously been available. I think it would be very hard, if not impossible, for someone to make the case that the Windows 10 start menu is better. From a sheer usability perspective, it objectively has less functionality while using substantially more resources and screen space.
Windows 10 is still a work in progress. Hopefully the problems in this article will get addressed before release. For those who make that charge, I will remind them that when Start8 came out, people made that same argument – Windows 8 was still in beta and they would “surely” add the start menu back before release. They didn’t.