-Long hours - if you're considering applying to or accepting an offer from MaPS, you need to be the type of person who is willing to put in over 50 hours a week consistently, and at busy times more than 60 hours (though these weeks are rare for most MaPSters). People who are successful at MaPS are motivated to do this for the sake of the quality of their work, and for their teammates. My personal perspective is that the time you invest in your career early on will pay off later. Be honest with yourself about your ability to do this consistently before coming here.
-Low to average compensation at junior to lower mid levels - Self-explanatory. It sucks, but this is a similar concept to the long hours in that successful people will look at their compensation early in their career as secondary to the Pros I've listed above. You're probably young and not supporting anyone - compensation doesn't need to be a lot right now. If you are successful at MaPS, eventually the compensation will come if you stick around, or at another job. As you are successful and get promoted, there will be significant jumps in salary.
-Sometimes this job can be very stressful. Client services means strict timelines, expectations for the highest quality work, and sometimes unreasonable demands. If you're the type of person who likes to be challenged, or can come out on the other side of that and feel good about it, this will work for you (at least for as long as you want it to).
-Staffing structure - You don't have much of a say in your projects; your placement is way more the result of the needs of the project teams and their managers rather than the needs of your development. Projects can vary pretty widely in terms of peoples' satisfaction with them and the hours they need to put into them - there are some projects that are truly awful.
You will not be proactively moved off of tracking or long-term projects until years have passed, especially if you become so valuable to the project that your manager doesn't want to lose you. On the bright side if you're happy with the project this can be a good thing. Hopefully they'll look at adjusting this system in the future, but in the meantime the best advice I have for new hires is to advocate for yourself. If you're really unhappy with one of or both of your projects and you've given it at least 6 months, first, try to see if there are ways you can improve what you don't like about it. Then, speak up for yourself with the staffer, and don't stop checking in about it until it's changed.
-Related to the above, variability in who you work with - There are some bad apples at MaPS who can cause a stressful burden on the rest of their project teams due to a poor attitude or poor quality work. These people are definitely rare at MaPS, but it can make your life difficult if you're unlucky enough to be staffed with one. Sometimes they are new people who are a poor fit, and I recognize many of those people behind some of the particularly nasty reviews on here. (This shouldn't be a surprise to those who are smart enough to see through the most verbose and pseudo-intellectual review. A principal-agent problem results when an agent who is supposed to be doing work to represent the interests of a principal but represents his own interests instead - it is classically applied to the dynamics between the shareholders of a company and the company's employees, or voters and politicans. It does not at all apply to conflicting interests between senior and junior people at a company. That review is a perfect reflection of that person's work quality, communication style, and attitude while at MaPS. It continues to be a running joke among MaPSters.)
Sometimes the bad apples are more senior people who were hired at a senior level, rather than working their way up at MaPS - and it shows in their work and interactions with junior staff. However, there's a review process at MaPS and the most disastrous performers leave or are shown the door eventually. This happens quickly at the junior levels but the more senior ones are sticky. The best advice I have for this if you happen to be staffed with one: try your best to work around it, and be honest when it's your time to review that person.
-Turnover is a challenge. MaPS has a difficult time keeping people around after they've put in a couple years. This can be a damper on morale and on project teams affected. MaPS really needs to address retention if they want to grow long-term.